How serial resistors actually reduce EMI?
Well one it slows down the rise time of your signal which reduces the high frequency content.  So that could help you if you didn't need your edges to be that fast.

It also lowers the current flowing through the trace and back through its return path which would lower the strength of the field created around it (and radiated out).

I would add that I've often seen this added lazily, or perhaps by the "emi group" after the design is done.  It's fine if you didn't need the edge rates but if you're trying to go fast slowing down your edges or destroying your eye may not be what you want.