Understandable how Simpson missed it on a perception level because a) the contact was on his blind side and b) he would have been expecting the Sunderland player to be the one flying in because of the way the ball was rolling.
But the fourth official has a free line of sight and HAS(!!!) to detect and see this. Is EXTREMELY frustrating how FA / PGMOL use fourth officials, they have an important role to play in terms of detecting incidents on the FoP, but here it is treated as an irrelevance.
Top SG1s fulfil the role the day before/after a game as if they literally just need someone to stand there. I just finished every game at WC86 - the reserve linesmen (as they were called then) essentially just sat down in a chair and held up the subs board when required. Besides sitting down, it seems FA / PGMOL interpret the role exactly the same today :/.