Going slightly against the grain but I’d say ESH.

I’m also atheist and also an introvert who prefers to spend my down time alone. But... Christmas is a big deal to my family, so I’m willing to make an effort for them. If you don’t want to celebrate Christmas or exchange gifts that is entirely your choice but I would say you’re slightly the arsehole for not respecting their beliefs/wishes.

The reason I’ve gone ESH is because your family also didn’t respect your wishes not to celebrate. It sounds like you didn’t communicate your wishes clearly in advance as they still expected you to show up and exchange gifts.

I would suggest in future sit them down calmly and explain why you don’t want to exchange gifts or party. But I’d also consider sucking it up for a few hours. Last year none of my family exchanged gifts outside of immediate relatives i.e. my sisters and I exchanged gifts but didn’t get/give any to aunts/cousins etc. But this was by mutual agreement which we discussed before Christmas. My family loves celebrating whereas I prefer more downtime like you. So I was at my sisters house by 9 and left by 4pm. We exchanged gifts, had food, sang carols etc and I still got a quiet evening alone.

You are not an arsehole for not celebrating Christmas or wanting to exchange gifts but I think you are a slight arsehole for not respecting your families beliefs and wishes. Perhaps next year communicate fully, in advance, that you don’t want to do gifts. Or agree to a set limit/secret Santa type situation. And perhaps suck it up for a few hours at the family gathering.