I guessed that the idea Mo was a messenger for all people came from the hadith where Mo claims there will be no more messengers after him, and surely an all loving god wouldn't leave everyone else out so he must have been for everyone. Plus, Islam will dominate the world, Allah (satta watta Allah) promises it in the Quran - and I think many people think "Islam = Muslims = Muhammad" and conclude again that therefore the teachings of Mo will dominate the world.
I've heard it before, but I remain unconvinced.
For example, if Mo was to be the last prophet, there are limited reasons as to why:
1) The bible contains the book of revelations (the conclusion)
2) The Quran would contain a repeat of Genesis and Exodus stories
(Or ofcourse why 'allah' would take stories taken from pagan religions and pass it off as his own)
I think the idea of him being the last prophet with the last book (aswell as 'islam will dominate the world') is most likely a marketing scheme, possibly constructed by his wife.
It's commonplace to jump on the back of popular religions, just as much as its easy to convince people to follow something, when you can make a long and whimsical prophecy as to how things will end up (both for an afterlife and with the religion).