It has less to do with gear and more to do with what you're used to.

We ride in sub-zero temps here in Michigan regularly. We have little choice.

I don't wear booties when it's above 25F.
I've never used those charcoal foot warmer things.

Today, for example, it was 22. I wore medium-weight tights. A base layer, a jersey, and a vest.

Hardcore is a relative term.
Up north, they probably wear even less.