It wasn't Mohammed who founded Islam.

[Rayaan]

And then you get to Abd al-Malik, the official founder of Islam, right?

So all those references of an army general named Muhammad and start from the year 634 CE is actually Abd al-Malik, even though he was born in 646 CE?

Why don't you please enlighten me on this guy a little more, if you know anything else.

So, apart from Sebeos, who's giving us a second hand account (at best) of a leader implementing rules and regulations, you have a military leader.

So... yeah... it is possible there was a military leader behind, at least, some of the arab conquests.... and it is possible such a person's name was Muhamad, or Mehmet, or something similar...
It is also likely that such conquests were done, not by a single army, but by several... well, several battalions, as we'd call them nowadays, huh?
This would give us several military leaders... if you get my drift?...

And exactly how did you go from "a military leader" to "several military leaders"? And where is the historical evidence for those several military leaders who lived around 634 CE, like the ones we have for Muhammad?

"It is just very likely" is not a decisive answer, by the way.