Why not? Shakespeare was a Muslim (and so was Hamlet). Abraham Lincoln was a Muslim. Christopher Columbus was a multiculturalist, heavily influenced by Islam. Even D'Artagnan of the Three Musketeers converted to Islam. So why not Da Vinci?
Both Islamic supremacists and non-Muslim multiculturalist dhimmis promote this nonsense, albeit for vastly different reasons: the non-Muslims think that by falsifying history and exaggerating the achievements of Muslims and the legacy of Islam, they can show Muslims that they're welcome in the West, ease the concerns that more sensible non-Muslims have about jihad and Islamization, and prove that they're not "racist," "bigoted" and "Islamophobic." Islamic supremacists, meanwhile, think that these claims advance and demonstrate the superiority of Islam and of Muslims as the "best of nations" (Qur'an 3:110).
Meanwhile, no one is going around claiming that Rumi was actually an Assyrian Christian, or that Suleiman the Magnificent was secretly Greek Orthodox. It manifests a deep cultural inferiority complex to persist in claiming the great figures of other cultures as one's own.
"Da Vinci was a Muslim?," from the Ahlul Bayt News Agency, April 9 (thanks to David):
(Ahlul Bayt News Agency) - Morteza Khalaj Amirhosseini's book "Leonardo Da Vinci's Drawings" contains best drawings as well as a detailed biography of this eminent artist. Based on valid sources, the book proves that Da Vinci had been converted into Persian.
Amirhosseini added: "I have prepared the book in order to address the needs of art students as there was no comprehensive book of Da Vinci's works available in Iran. We should know an artist by his works, but unfortunately Da Vinci is just an icon in Iran with mythological fame."
Amirhosseini went on to say that the book presents a complete biography of Da Vinci in which he has proved based on first-hand sources that the Renaissance artist had become a Muslim. However, the west prefers to keep silent on the subject, he added.
He added: "A French writer in the 19th century has evaluated the issue of Da Vinci's conversion to Islam in a treatise, but the west has banned the publication of this treatise."
Pure fantasy. The modern West would like nothing better than to trumpet such a claim, right alongside similar fiction such as the 1001 Muslim inventions exhibit.