[tyingq]

Not caching the questions and answers makes sense to me, as I imagine the hit rate wouldn't be terribly good. I would guess, though, that they somehow cache things like the sidebar list of blog articles, featured items, "Hot Network Questions", etc.

[banana_giraffe]

They do in fact cache some things like that, they've had caching issues in the past (and again recently, I think) with the wrong cache being used in some situations:

https://meta.stackexchange.com/a/235277