Whether Boris attended the party or not (if you're giving him benefit of the doubt), it took place at his home. Which is, as Rabb loves to point out, also his office.
If he wasn't there then he can prove that. Carrie probably was at home at number 10 as she'd not long had a baby, we were in lockdown and there was not many other places she could have gone. If she wasn't there then their house or office staff were.
So where did his private secretary get permission for the party from if not from him or her? You surely don't invite people into the back garden of your boss's house without him or his wife knowing you were going to do it.
This didn't take place at some obscure park. This took place in the back garden of the PM home. No-one hovers around those grounds with permission, I would imagine.
So attending or not, Boris and Carrie knew it was happening, did nothing and are now lying.
All of which is immaterial. If Boris didn't attend and could prove he was elsewhere (which as PM is pretty easy as visits/meetings are documented), he just has to say he didn't attend. But he won't say that because he was there. And he knows a photo will surface if he says he wasn't.