Even if they don't agree with their interpretation on the handball itself, they can't argue that they shouldn't have also checked for an offside in the build up. They never said it wasn't offside, they said it wasn't the reason they dismissed it.
I don't even think we need lines to see Richarlison was offside:
This is a split second before he strikes the ball:
View attachment 37328
and a split second after:
View attachment 37330
Personally, I think he looks offside in both frames.
So, at worst it's the right call for the wrong reason. I've even seen someone claiming "there was a throw in before it happened so it couldn't have been offside", which is just bizarre and shows they probably know it was offside.