Tom Holland does believe in a historical Muhammed. There's a difference between thinking the Hadith, the quran and Islam is a product of its time and place and then arguing that Muhammed doesn't exist.

Hollands argument, is that the reason we cannot find written testimony from Muslims in the first early decades after Muhammad’s death was because Muslims yet exist. The religion of Islam is a later invention used to pull the arab empite together. However as multiple historical sources clearly mention Muhammad his existence isn't in doubt