The thing is, if it's a Poppi QTr reference:
1) Why QTr? Poppi Alpha is much more well-know then any of her other forms & is her main form in the story most of the time. (in fact, QTr is completely optional)
2) Why put Pyra, in her default color & Mythra fighting on Spear Pilliar? Isn't a bit random to show a alt that is referencing a party member like Tora & his blade?
3) Why is all the crystal on Mythra body pure red? Poppi in all of her forms have orange crystal/lines, which if they were referencing, surely it wouldn't be hard at all.
4) Would Sakurai & Monolith really allow such blatant errors like this? Like yeah, we can show exemples of alt that mean nothing on other characters, but all of Shulk alt reference a party member to the teeths (beside of course his swimsuit alt), they even changed the hair colors for each alt in Ultimate. Meaning they wanted to reinforce the references even further they already did.
Idk, I just can't buy that it's Poppi atm. Plus Weltall is kind of a big deal to the story of Xenogear, as it's the mecha of the main character. So it's not like they picked a random mech to reference.
I will say it's definitly a obscure reference if that's the case, but hey, I kinda dig that tbh. Especially since XB2 as alot of similar elements to Xenogear in place.