Tom makes the claim that after reviewing Christian sources, they admit that the conquering Arabs of Jerusalem took over by peaceful negotiations. However, the Arabs never claim to be in a particular religion. Not even Islam. They only say they are believers. Even the ruler Muawiyah 30 years later never claims to be a Muslim or mentions Muhammad (saw) in these sources. Also, the earliest coin after Muhammad's (saw) death with his name on it is dated to be 60 years after his death. He also notices that there are stories mentioned in the Qur'an from the Bible, and that Mecca is not mentioned in the Qur'an. Therefore, he concludes that the Qur'an and Islam was invented much later by the Arabs when they became rich and wealthy after conquest and they purposely made Mecca the origin of revelation since it is a barren land with no history, so it was easier to make up history in the area.
This however, is a bias and logically flawed approach in studying the origin of Islam. For he only chooses to look at Christian and non-Muslim sources and not the sources of Muslims. Therefore, you cannot conclude a historical account of a religion without at least viewing all other sources, which would include the Muslim sources itself and instead only refer to specific sources. At the same time, he compares what he finds within these outside sources, with what is mentioned in the Qur'an, which is a Muslim source.
This is cherry-picked research. Notice, he rejects almost all Muslim sources in the beginning, then he uses the Qur'an later, which is a Muslim source, to draw his conclusions. So his historical account is not valid, because cherry-picked research is bias, rather than objective. He also makes the claim that because the name Muhammad or the word Islam is not in these early non-Muslim sources and mentioned many years later after Muhammad's death, this means that Islam was invented much later by the Arabs. Again, such a claim is logically flawed. For again, he does not look at Muslim sources, such as the hadith and works of early Muslims, who do in fact document his name.
So the research by Tom Holland is invalid and cannot be utilized to conclude that historically, Islam was invented by the Arabs much later after Muhammad's death.
To the contrary, his admission that the Arabs conquered Jerusalem by peaceful negotiations shows that Islam was not spread by force, and the absence of historical records would support divine revelation since a miracle is beyond natural cause and effect, and the historical method only applies natural cause and effect.