I don't know enough to answer that question. I don't actually know the details of what PSBTs can specify or how they are signed. I was just going off of the what the link said.
Discussion
Looking deeper into the article, it seems like promo for BitBox. And it's weird because they try to boost their device and software saying they're more secure because of their Anti-Klepto feature. But really it's even better to use hardware that doesn't use companion software and never even touches the software directly. For example, a Coldcard used with Sparrow
Definitely true it’s a promo for Bitbox. But the attack vector still looks real. Looks like those defenses do nothing against a malicious RNG.