I don't think he's underestimating the UX/design piece, I think it's just outside of the scope of what he's talking about. It seems pretty clear to me that his statement is talking about the underlying AI technology (which makes sense given his background).
And in any case, I wouldn't call anything about the UX or design here innovative - it's obviously a huge improvement in terms of usability, but a chat UI is a pretty well-established thing.
I think the idea of a "prompt" is actually pretty cool. I never saw that framing prior to GPT-3 and I think it reframes the entire idea behind what a model does and how you interact with it.
And in any case, I wouldn't call anything about the UX or design here innovative - it's obviously a huge improvement in terms of usability, but a chat UI is a pretty well-established thing.