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After looking into and debating this with a bunch of friends I'm starting to think a lot of modern theropod skull reconstructions are wrong. Most of that thought process can be attributed to distortion and tooth over insertion.
www.tandfonline.com/doi/full/1…
Rex does align quite close to alligators in the study, but I should stress one thing from either side of the argument:
1. There are no squamates scanned in any studies that I am aware of (which is a massive missed opportunity and a huge lack of data in this equation.)
2. Rex's jaw is very different from squamates. It's entirely probable that if rex had lips they would be quite different from squamates.
Ultimately, the large argument to whether they would have lips or a alligator (specifically) like mouth depends entirely on how far they can close their mouths, and how long their teeth are. If the teeth were to cross past the foramina line at max closure then rex very likely would be punching through it's own lips when biting down hard. There are also divots in the palate suggesting theropods might have kept their mouths fairly closed.
And before anyone says "but it's teeph will dry ouut" There are crocodiles that hibernate on land for four months, Birds that can't fully close their own mouths, and animals that have evolved ways of keeping their teeth intact while having them outside their mouths exposed to the air. Not to mention that while rex has a slower tooth replacement than most theropods, it also replaces a full set in a month and the entirety of the lower jaw's teeth would be covered by the upper jaw's gum line (assuming alligator like condition on the face, which is a lot more plausible than other crocodylomorphs).
if anyone has any really good arguments to bring to this feel free, but I still feel that this is still an unknown.
I feel that any land-based animal would have lips. Dinosaurs might have been related to birds, but a lot of their features resemble that more of reptiles and while a lot of dinosaurs did have bird features it was more in places like the arms, legs and sometimes hips. a lot of theropod dinosaurs were feathered but tyrannosaurus probably wasn't, even though it was related to birds. I do also see your point in that crocodiles and things like that hibernate on land but they still water based while rex was not that. Also, tyrannosaurus teeth on a lot of skulls found were probably very misleading, after the tyrannosaurus died the teeth slowly fell out just a little making it seem that it had SUPER big teeth that lips couldn't cover. I just find it extremely unlikely for tyrannosaurus specifically to not have lips. Comparing dinosaur lips with birds that can't fully close their mouth is like comparing a platypus jaw to something like a rat, like they might be related but they were very different. Also, birds do not have teeth, and even when they have teeth like structures such as geese their teeth covered over by their beak. This is my last part of my argument, tyrannosaurus had to hunt things with armor which if they had no lips to keep their teeth protected while not hunting the teeth would definitely be brittle enough to break under the strength of something like a triceratops frill or even an ankylosaurus osteoderm.