I think that Mat is a Hero Reborn because of something i remember reading on either this page or someone else's page. It said something about how RJ gives hints about stuff that will happen in the books by writing in small, seemingly unimportant remarks. such as how siuan sanche said how she'd be stilled if the tower ever found out what she was doing with Rand and the rest. I found one such remark in the tEotW(pg 278). It says... (Perrin)"I'd think you(Mat) would like that- being a king and a hero reborn." I think that this is one of those hints that RJ likes to give, and proves that Mat is a Hero Reborn.