I think the problem that I'm sure Millwall fans see is that yes the man is standing in an offside position but has him being them stopped the GK from seeing the ball the whole way. He appears to see the ball the whole way and saves it . He then spills it which isn't because off the Millwall player being there and then the other player taps home. I guess it depends which side you are on but Millwall were certainly on the wrong side of alot of decisions this afternoon
Watching clip on twitter, the millwall player is definately in the line of the vision of the goalkeeper and thus interfering with player so the ‘goal’ was quite correctly disallowed.
Great teamwork by team ref i would say.