What's he supposed to do? If the linesman was actually watching them and is convinced he saw a foul, the ref (who was looking elsewhere) is hardly gonna tell him he was wrong.
Especially when said lino got his flagging wrong in the initial instance, showing even he was confused by what he saw. I mean, him flagging for a free kick suggests he saw Robinson lash out at Hunt. But then somewhere in his mind, he remembered Hunt fouled Robinson, leading to the penalty award. That level of confusion, at 0-0 at that stage of the game? And the ref goes with it? Nah.
I'm not saying the ref should disagree, I'm saying he shouldn't award the 2 major decisions based on a confused obscured view of what his assistant thinks happened.