Is there a reason they're using human brain cells specifically? This seems like it would also work with neurons from other creatures.

I was under the impression that the relative intelligence of humans versus other animals was largely a function of brain cell quantity, not quality. Can 200k human brain cells really learn faster than 200k mouse brain cells?

A more cynical take is that they're just using human brain cells for shock value. They chose DOOM because of the "can it run DOOM" meme, so they clearly value publicity a lot.