I mean he kind of goes into it in the video like he explains something about how usually skin coverings don't change to fast from ancient relatives (its been a while and I'm saying all of this off the top of my head)
I for sure will check out the video it sounds interesting
I think I'm actually going to have to do an update on this drawing because there was a new finding that I think proved that at least the complete head of T-rex was scaly (Im not 100% on that)