He says he voiced him in the "trailer" and "in the game for the parts that Chrom is in it."
I expect he's seen the trailer and knows the difference there.
Yeah, Matt Mercer knows what he's talking about as far as Smash goes. Dude's a huge geek, made There Will be Brawl - he even talks about how Robin has a unique fighting style and how Lucina is a good choice since there's already a lot of male characters.
Here's a rough transcript of what he says:
"Speaking of Smash Bros., the fact they revealed Chrom as kinda being in the game I'm really happy about. I voiced him in the trailer and in the game... for the parts that Chrom is in it... and I'm kinda excited because there are parts of the story that kinda deal with Chrom not happy that he's not actually in the game."
Definitely sounds like a lot more than "I guess I'll have my chance another day" and generic Final Smash stuff. Whether it's a full-fledged "story mode" or just some Codec-style conversations or whatever, who knows. Maybe there's an Adventure Mode with post-game cutscenes like Marvel vs. Capcom and stuff.