Sikhism is a rather new religion, founded by Guru Nanak (1469-1539 AD) in the 16th century. Today, it is still mostly followed by those originating from the Punjab state in India, where it was founded.
In order for a new religious movement to be compelling, it must attempt to explain why it only came along rather recently in history. Most often, this is done by attempting to absorb other, older religious traditions.
Sikhism, in this respect, cast a wide net, and took the approach of "all religious traditions contain the essential elements of what we teach", while simultaneously denying all of them. This led to the inconsistent affirmation of many religious scriptures in their main scripture, the Guru Granth Sahib.
For instance, it ascribes the creation of the Vedas, and the Puranas, to God:
Guru Granth Sahib - Bilaaval, First Mehl, T'hitee - The Lunar Days, Tenth House
2 The True Lord Himself created the universe, joining the elements together. Breaking the cosmic egg, He united, and separated. He made the earth and the sky into places to live. He created day and night, fear and love. The One who created the Creation, also watches over it. There is no other Creator Lord.
3 The Third Day: He created Brahma, Vishnu and Shiva, the gods, goddesses and various manifestations. The lights and forms cannot be counted. The One who fashioned them, knows their value. He evaluates them, and totally pervades them. Who is close, and who is far away?
4 The Fourth Day: He created the four Vedas, the four sources of creation, and distinct forms of speech. He created the eighteen Puraanas, the six Shaastras and the three qualities. He alone understands, whom the Lord causes to understand. One who overcomes the three qualities, dwells in the fourth state. Prays Nanak, I am his slave.
Guru Granth Sahib - Maru, Fifth Mehl, Fourth House
1 The One Universal Creator Lord created the creation. He made all the days and the nights. The forests, meadows, three worlds, water, the four Vedas, the four sources of creation, the countries, the continents and all the worlds, have all come from the One Word of the Lord.
Guru Granth Sahib - Raag Gond, The Word of Kabeer Jee, Second House
2 This bride is so beautiful. The bells on her ankles make soft music. As long as there is the breath of life in the man, she remains attached to him. But when it is no more, she quickly gets up and departs, bare-footed.
3 This bride has conquered the three worlds. The eighteen Puraanas and the sacred shrines of pilgrimage love her as well. She pierced the hearts of Brahma, Shiva and Vishnu. She destroyed the great emperors and kings of the world.
Anyone who has read the Vedas and Puranas, however, knows that they are fundamentally at odds with Sikhism. The Puranas teach many strange and horrible things, including widow burning (Sati), and a rigid and discriminatory Caste System. The Vedas include the Upanishads, which teach a Non-Dualistic view of the universe, where the soul of all beings is one and the same with the ultimate reality, Brahman. These things are utterly rejected by Sikhism, which is Monotheistic, rejects Sati, and abhors the Caste System. So, the Guru Granth Sahib cannot logically ascribe the creation of the Hindu scriptures to God, without creating contradiction.
Likewise, the 18 Puranas contradict each other. The Shiva Purana teaches that Vishnu got tricked by Brahma (Shiva Purana 1.7.26-33), and that Shiva is the supreme Brahman, whereas many other Puranas teach that Shiva and Vishnu are the same, or that Vishnu is the supreme Brahman. The Padma Purana teaches that the Matsya, Kurma, Linga, Shiva, Skanda, and Agni Puranas are vicious, and lead one to Hell (Padma Purana 6.236.18-22). There are many other fundamental contradictions between the Puranas, depending on which deity they are emphasizing.
The Quran and the Bible, also, are given a form of tacit approval in the Guru Granth Sahib:
Guru Granth Sahib - Basant, The Word of the Devotees, Kabeer Jee, First House
1 The earth is in bloom, and the sky is in bloom. Each and every heart has blossomed forth, and the soul is illumined. My Sovereign Lord King blossoms forth in countless ways. Wherever I look, I see Him there pervading. The four Vedas blossom forth in duality. The Simritees blossom forth, along with the Koran and the Bible.
2 Shiva blossoms forth in Yoga and meditation. Kabeer's Lord and Master pervades in all alike.
Guru Granth Sahib - Maru, Solhas, Fifth Mehl
3 Let your practice be to live the spiritual life. Let your spiritual cleansing be to renounce the world and seek God. Let control of the mind be your spiritual wisdom, O holy man; meeting with God, you shall never die again.
4 Practice within your heart the teachings of the Koran and the Bible; restrain the ten sensory organs from straying into evil. Tie up the five demons of desire with faith, charity and contentment, and you shall be acceptable.
5 Let compassion be your Mecca, and the dust of the feet of the holy your fast. Let Paradise be your practice of the Prophet's Word. God is the beauty, the light and the fragrance. Meditation on Allah is the secluded meditation chamber.
Guru Granth Sahib - Bibhaas, Prabhaatee, The Word of Devotee Kabeer Jee
1 Prabhaatee: Do not say that the Vedas, the Bible and the Koran are false. Those who do not contemplate them are false. You say that the One Lord is in all, so why do you kill chickens?
Of course, the Quran and the Bible are utterly at odds with the Vedas, the Puranas, and each other. So this approbation is nonsensical.
At the same time, one can find very many statements in the Guru Granth Sahib which almost appear to mock the Vedas, Puranas, Quran, and Bible, and deny their ultimate authority. "Affirming" scriptures only to the extent that they happen to line up with what one already believes is completely meaningless. It proves nothing, and lends no credibility whatsoever to whoever is engaging in such a practice, because it can be done by anybody in absolutely any other religion as well.
Sikhism is a very new religion, which tries to substantiate itself by inconsistently appealing to prior scriptures and religions. However, as is the case with all syncretic religions, this only leads to logical contradiction.