How can you say that Prophet of Islam was a Historical Personality?

There are two references of secular historians which show that the Prophet Muhammad was a historical figure. These are:

1. The French philologist Ernest Renan wrote in 1851 that, unlike the other founders of major religions, the Prophet Muhammad was "born in the full light of history". (Bernard Lewis, The Arabs in History, p.36 )

2. In his book, The Decline and Fall of the Roman Empire (pp. 79-80), the historian Edward Gibbon while speaking about the Roman king Heraclius, makes mention of the Prophet Muhammad. He says that Mahomet, an obscure citizen of Makkah, had predicted the victory of the Romans over the Persian Sassanid empire in the first quarter of the seventh century. Here, it is clear that Gibbon recognizes the Prophet Muhammad as a contemporary of Heraclius. And, Heraclius is accepted as a historical figure. Therefore the Prophet Muhammad, being a contemporary of Heraclius, should also be regarded as a historical figure.