I guess he's relying on his definition of "party". I went to a retirement do last week - a few drinks and a buffet in a pub function room - but it's not something I would call a "party". Nowhere in the invite was the word "party" used. When Lady T asked me where I was going, I didn't use the word "party".
Semantics, maybe, but I assume that's what Johnson is relying upon in his defence.
(To me, a "party" is something that's involves activities such as music, dancing, DJ, balloons, party poppers, party games, dressing up, etc)