If so, perhaps human brains will evolve to become even smaller, with less stand-alone memory.

Seems like that idea is based on this article claiming the brain has shrunk over recent millennia which in turn has resulted in a commentary questioning the inferred reasons for this shrinking.

Lots of speculation, in my uninformed view.
It feels a bit of a stretch that AI will decrease our brain size even further.

I think social media-induced brain rot will have a bigger impact, if it already hasn't.