The one thing McDonnell was spot on is that whatever way they went they would be alienating a part of their vote. By saying they'll get Brexit done too and with a younger support base (just look at the maps that show the votes by age) that mostly wanted to stay in the EU the result would have likely been just as bad, as those voters might have jumped to the Lib Dems.
Labour may have won the traditional seats but they would have plummeted in the rest.
To be fair they recognised that, which is why they tried to play it down the middle. I maintain that Corbyn's Brexit position made perfect sense - the problem was the complete lack of ability to communicate it, because of how tragically inept Corbyn and his top team are.
Brexit is the smokescreen to the bigger problem though - as one MP said (can't remember who), on the doorstep for every one voter whose main problem was Brexit, there were five whose main problem was Corbyn and Momentum.
If Labour had come down firmly either on the side of leave with a deal or remain, they'd have still had that problem. Leavers would prefer Johnson to Corbyn and trust him more to deliver it, and Remainers like myself simply couldn't vote for Corbyn anyway even if it meant remaining.
The problem comes back to Corbyn and Momentum. They were and will continue to be electorally toxic, and there's still a massive misunderstanding and outright excuse making about why that election was lost.