I don't know where you get that. Most of the Arabian peninsula had converted to his islam by the time of his death.
You have only their word for it ~ and he was long dead before they came roaring out into the middle of a massive worldwide depression of the very worst kind ~ and discovered the Byzantine Army and Navy hadn't even been paid for the last 100 years!
Most serious analysts take note of the fact the Arabs had gold to pay (Petra had a whale of a lot of gold) and they literally bought existing armies to do their bidding in their sweep across North Africa.