Of course, the assistant referee should still have seen that Mane was offside. It is not even a close call, and we should be thankful that the final score was 4-0 and that the error did not impact the result of the game. It is a comfortable decision which you would expect an elite assistant referee to get right 10 times out of 10.
But he is a human being, and human beings do see things differently and they do lose concentration. It is a difficult job but we saw in the World Cup how it is possible to make these decisions easier for the assistants….