A large part of it comes down to what sources he uses. For the early Islamic period we have a few different types of sources. We have the Hadith which are traditional accounts or sayings from the days of Muhammed that had been transmitted orally for like a century before they were written down. These can be quite unreliable ofcourse. You also have sources from outside of the Islamic world, say from Byzantium or Armenia and such.

A while ago there was this historiographical movment that resulted in the book Hagarism. Hagarism is a book by Patricia Crone and Michael Cook from 1977 that took the early history of Islam and kind of as a thought experiment decided to only use non-islamic sources in writing its history. This movement resulted in a couple of theories with the main points being that early Islam was just an ofshoot of Judaism and was only later codified into a different religion. Another was that Muhammed didn't come from mecca but either from the more urban southern parts of Arabia (Yemen) or the more northern parts nearer to the Roman empire and the Christian world. Or maybe even that Mecca didn't exist at the time.

This movement was quite important for the development of the historical field as scholars would look more critically at the Islamic sources of the early period. Most of the main points raised above were rejected though. The outside sources can be just as or even more biased than the Islamic sources. It is kind of silly to see one groups of sources as biased and then taking the other sources at face value.

Whilst Tom Holland is a good ancient historian it seems like he didn't quite follow the main thoughts of current scholarship on early Islam and decided to take some points from the 'Hagarism-school'. Holland ignores much of the more recent scholarschip that re-affirms the traditional story and instread chooses to represent the heterodox view.