Lots of the Very Online Left seem to think that they can genuinely beat Starmer.
The most hopeful precedent they have for that is Galloway winning Bethnal Green and Bow in 2005, but that's not really much of a precedent. The swing was 26.2%, which wouldn't be enough to stop Starmer winning. Galloway had an existing public profile and strong campaigning skills that Andrew Feinstein is unlikely to match. It's a different constituency- Galloway picked a constituency to get elected in, Feinstein is picking an opponent (he'd have chosen another constituency if he was prioritising getting elected). And there was a substantial vote in Labour's overall voteshare which isn't going to happen this time. And there are Greens running to split the left of Labour vote- a few people might think the Greens were a nicer option than Stop The War.
They think if they all turn out, they'll persuade everybody. Like nobody else has ever thought of taking out the top man before. The Lib Dems in 2005 wasted a ton of effort on "decapitating" Michael Howard, Oliver Letwin and David Davis.