It still sounds like something Spock would say

Here's the thing though... it isn't factually incorrect, and taken together with how Ethan Peck has played the character, this line could just be viewed as Spock's attempt at a joke.

I'm not going to say who they are, let's just say it's some asshole who bangs my mom

Relevant xkcd: https://xkcd.com/1368/

Couldn't that be cannonically explained as Spock being secretive about his past?

Perhaps both things are true. If Sarek also had a human ancestor it would explain quite a bit about him.

Who knows, maybe he's also related to the Vulcan that stayed behind in Carbon Creek