If the xG metric can't factor in if a goal chance is offside, is there any point to it?
It's not that it "can't", it's that Robinson's shot is a valid shot because it was a valid shot. It wasn't flagged offside so it counts. As I said, it it went in, it would have been checked.
And Atomic, so many stats are putting a number of something you "know". Literally every stat in the world is putting a number to something, but sometimes being able to do that is extremely helpful. It's some sort of silver bullet which is going to make everything else obsolete, but it's the most predictive single stat for a team's performance, and is very informative too.