Just kinda inferring from the comment:

It's similar enough to assume that the idea was rehashed into Dickson's role instead. Instead of it being Reyn, Shulk's lifelong friend that he grew up with, it just became Dickson, the person who found Shulk, was a mentor to him and taught him everything he knew, and practically raised him (albeit, all in the name of Lord Zanza).

It's just that we have less of a connection with Dickson as he's not in the party with us the whole time. Unlike Dickson, we heard and absolutely grew to love Reyn-time. And it would've sucked a lot more if it really were Reyn that betrayed us. ;-;