I think it's more than just the trailer footage. It's also past precedent that leads me to say they'll be upper high tier at least. For one, being able to be a fast, combo-centric character when it's optimal, then switch to a character with high survivability and kill power when that's optimal, has proven to be a very good thing to have (PT). I doubt it'll be as extreme as Squirtle and Zard, but is it a point in their favor? Certainly.
Bats Within is one of Bayo's greatest strengths. I'd reckon every other character in the game would love to have it. Although, we can't be certain that's what P&M have. We do need clarification on this one. But if that is what's going on, or even if it's activated differently but is practically the same, that's a big strength right there.
But honestly? Those alone aren't the biggest reasons I'm thinking they'll be really good. The biggest reason is simply that Myhtra looks pretty fast, and Pyra doesn't exactly look sluggish either. So, where do all the fast swordies place on the tier list? Very highly. I mean, we've got Cloud and Sephiroth. We know Cloud has a frim place in high tier, and with how much Sephiroth improves offline, Sephiroth should be there too at the very least. Then we've got Chrom, Roy, and Lucina, who make good arguments for top tier. And lastly, Shulk, who would probably be top tier without Arts, but has Arts to make him stupid good. It also seems like they're going to have big swings for their aerials. I think one thing holding back Byleth is they didn't get fairs and bairs that cover a ton of space. Doesn't seem to be the case for P&M. And when a slow sword character can cover a ton of space with their aerials, where do they end up? Well lookie there, Corrin and Ike are high tier! And Mythra looks like she'll be quite faster than them, with Pyra probably outspeeding as well.
So, I really don't feel like I'm coming from nowhere when I say they'll probably be upper high tier at minimum. This is a guess with a lot of education behind it. It's more than just a first impressions thing. And if I'm wrong, oh well. That's the nature of speculation. They'll be dwelling in the depths of low miditer and life will go on.