Chuka cares about one thing and that's becoming leader, it was evident right up to the point where he cancelled the function room for his quietly shelved leadership bid. Apparently Corbyn saving the party hasn't dissuaded him too much.
Saying Corbyn actively sabotaged remain is completely wrong, he campaigned just as hard as anyone, the one difference is he refused to share a platform with other parties he wanted to make a Labour Party case which he did and he toured the country on that platform but would he ever get the coverage the official campaigns did, no effing way. The message at the time was 'Corbyn is not fit to do his job' and people fell for it and people discredited everything he ever did.
This part isn't aimed at you this is just a general point:
It's also unfair to say 'young people' didn't understand his stance on Brexit, they're much smarter than your average voter, they actually read stuff.