The source citations are facts. We can check that Alan Kay "The Early History of Smalltalk" shows this on page 82:

"Unfortunately, inheritance — though an incredibly powerful technique — has turned out to be very difficult for novices (and even professionals) to deal with."

When the presenter tells us — 13:45 "he was already saying he kind of soured on it" — that is not a fact, it's speculation. That speculation does not seem to be supported by what follows in "The Early History of Smalltalk".

One page later — "There were a variety of strong desires for a real inheritance mechanism from Adele and me, from Larry Tesler, who was working on desktop publishing, and from the grad students." page 83

And "A word about inheritance. … By the time Smalltalk-76 came along, Dan Ingalls had come up with a scheme that was Simula-like in it's semantics but could be incrementally changed on the fly to be in accord with our goals of close interaction. I was not completely thrilled with it because it seemed that we needed a better theory about inheritance entirely (and still do). … But no comprehensive and clean multiple inheritance scheme appeared that was compelling enough to surmount Dan's original Simula-like design." page 84