Casey says he “didn’t really cover Alan Kay” https://youtu.be/wo84LFzx5nI?t=8651 To me that says that Kay wasn’t a major focus of his research. That seems to be reflected in the talk itself: I counted 6 Bjorne sources, 4 Alan Kay sources, 2 more related to Smalltalk, and about 10 focused on Sketchpad, Douglas Ross, and others. By source count, the talk is roughly 18% about Alan Kay and 27% about Smalltalk overall - not a huge part.

As far as the narrative, probably the clearest expression of Casey's thesis is at https://youtu.be/wo84LFzx5nI?t=6187 "Alan Kay had a degree in molecular biology. ... [he was] thinking of little tiny cells that communicate back and forth but which do not reach across into each other's domain to do different things. And so [he was certain that] that was the future of how we will engineer things. They're going to be like microorganisms where they're little things that we instance, and they'll just talk to each other. So everything will be built that way from the ground up." AFAICT the gist of this is true, Kay was indeed inspired by biological cells and that is why he emphasized message-passing so heavily. His undergraduate degree was in math + bio, not just bio, but close enough.

As far as specific discussion, Casey says, regarding a quote on inheritance: https://youtu.be/wo84LFzx5nI?t=843 "that's a little bit weird. I don't know. Maybe Alan Kay... will come to tell us what he actually was trying to say there exactly." So yeah, Casey has already admitted he has no understanding of Alan Kay's writings. I don't know what else you want.