The correlation between brain volume and intelligence is fairly weak. Neanderthals had larger brains than humans, for example. Looking outside the hominids, we have fairly smart corvids with relatively tiny brains.

That means the chain of thought “brains volume decreased, so individuals must have gotten less intelligent. Yet, societies grew smarter, so there must be herd intelligence” breaks at “so individuals must have gotten less intelligent”.

I think/guess that argument may have merit when replacing brain volume by number of neurons (https://en.wikipedia.org/wiki/List_of_animals_by_number_of_n...)