I agree, but emphasis: apparently. The press release says, specifically, "Uncover the truth and fight alongside your squad – as well as the cast from the original Mass Effect and Mass Effect 2." That sounds like you're fighting alongside them in the same way you fight alongside your squad, but it could potentially mean a hundred different things. You could argue you "fight alongside" Wrex and Mordin on Sur'Kesh. I don't think that's the case, I'd instinctively say they mean as squadmates, especially given the picture of Wrex and Miranda interspersed among your modern day squadmates, but... there's nothing out there that specifically says, "Wrex will be in your Mass Effect 3 squad," so, pinch of salt!
I hope he is, though. It feels wrong that ME3's the only game not to feature a krogan squadmate, it feels wrong that the series' signature krogan is only a companion for the first act, and it's a shame that Wrex is the only ME character you can't take with you for the final act. It's almost like he was demoted. This would correct all of these things.
The DLC becomes available after the Citadel invasion, for the record, so characters who've already had their endings by the end of the invasion are unlikely to return. This means Thane's almost certainly not there, and I doubt we'll be seeing much Mordin, either. Still pretty completionist, though - Zaeed's back, for instance! If they were going to brush over anyone, you might think they'd brush over a free download DLC companion like Zaeed, but no! Not the case. This DLC's one of the last roles his voice actor recorded before he died, this month, in fact. Very cool bloke. He'll be missed, and I can't wait to meet him again in March.