To your first objection, I agree that “the gradient may not be the same in the two,” when you are talking about chimp-to-human growth and human-to-superintelligence growth. But Eliezer’s stated reason mostly applies to the areas near human intelligence, as I said. There is no consensus on how far the “steep” area extends, so I think your doubt is justified.
Your second objection also sounds reasonable to me, but I don’t know enough about evolution to confidently endorse or dispute it. To me, this sounds similar to a point that Tim Tyler tries to make repeatedly in this sequence, but I haven’t investigated his views thoroughly. I believe his stance is as follows: since a human selects a mate using their brain, and intelligence is so necessary for human survival, and sexual organisms want to pick fit mates, there has been a nontrivial feedback loop caused by humans using their intelligence to be good at selecting intelligent mates. Do you endorse this? (I am not sure, myself.)