I haven't seen any highlights of this decision so I may be getting this wrong, but Elphick being off of the pitch doesn't mean he is not counted when an offside decision is being made, so if he is still goal side of the attacker along with the goalkeeper then it is not offside.
Interesting point - on which I can throw no light at all - but it reminds me of one of the Wolves goals in the 1960 Cup Final - which was claimed to be offside by Rovers as the defender playing the scorer onside was actually standing in the back of the net behind the goal-line - the ref pointed out that he had his hand on the goal-post - which is part of the field of play- GOAL!
I do remember some weird shit sometimes