Biden did maintain distance from Trump throughout the debate (although I think you should maintain distance from Trump regardless of pandemic). And he wears a mask regularly and so on. So I think he did take precautions.
Still I'd rest easier and join in on the schadenfreude if or when Biden gets a neg. test.
That said, Nancy Pelosi beaming with the grin of a Cheshire Cat is a thing I didn't thought I'd need to see. I hope she doesn't regret this.