In a sense humans are fancy autocomplete, too.

I actually don't disagree with this sentiment. The difference is we've optimised for autocompleting our way out of situations we currently don't have enough information to solve, and LLMs have gone the opposite direction of over-indexing on too much "autocomplete the thing based on current knowledge".

At this point I don't doubt that whatever human intelligence is, it's a computable function.

You know that language had to emerge at some point? LLMs can only do anything because they have been fed on human data. Humans actually had to collectively come up with languages /without/ anything to copy since there was a time before language.