I don’t think it’s because they were Brexiteer’s at all. I don’t think McDonnell and Corbyn felt overly strongly one way or the other. Getting the opportunity to enact socialist policy was clearly their main priority and Brexit was an annoying distraction.But as McDonnell and Corbyn were lifelong Brexiters they didn't want that, they wanted Brexit. They even tried to stifle debate on a second referendum in the Labour conference. I do think that with different Labour leadership Brexit might have been avoided, not for certain but there would have been a chance.
But no, according to some people here Corbyn wasn't a Brexiter and nothing was his fault even though he was leader, the blame is all Starmer's
It’s likely they wanted to stifle Brexit debate because they quite rightly, knew a remain position would be electoral suicide. Unfortunately, being the democrats that they are, they let the party decide on a disastrous Brexit position which was very costly.
How does a different Labour leader avoid Brexit? The number of constituencies in favour of leave made it impossible for a party with a remain or ‘people’s vote’ policy to win in a general election.