We've been here before, albeit with a different thing. Johnson has not once corrected the record at the Commons, meaning he has lied and the lie has been kept on the books, uncorrected. This obviously means he never "misspoke" and fully intends to stick to the lie. Starmer had a moment where he said something incorrect and got hauled up on it. As soon as it was pointed out, Starmer corrected the record. Do the Tory voters care? Not a bit. As far as they are concerned, Starmer is as big a liar as Johnson because of the and the fact Johnson has NEVER corrected the record whilst Starmer did simply doesn't factor into their thinking at all.
I think it will be the same here. If Starmer is fined and resigns, will Tory voters care that one leader stood by their word and actually did practice what they preach (resigning when found to have been in the wrong)? Of course not, they will point the finger with much enthusiasm claiming he's a complete hypocrite and no better, completely ignoring the very different results to being fined or the very different scale of the breaches.
I do agree that Starmer should resign if fined, his position becomes indefensible and I do not care to support someone who will be like Johnson to beat Johnson. Johnson is the absolute scraping the bottom of the barrel of lying, deceitful and lacking integrity politician in this country overseeing a wholly corrupt party and government. Those who still support him don't have any care for this, it's all down to "my side winning" at this stage. Better to attempt to win over those for whom integrity does still matter than to chase after the lost causes who will continue to vote Tory even if their policy was to fire nukes at ourselves.