Tl;dr: a vote would force all Tory MPs to choose sides and thus expose just how much his stock in the party has fallen.
However, other Tory MPs suspected Johnson’s stance was motivated by a desire to avoid exposing the thinness of his support in the Conservative parliamentary party.
Only 21 Conservative MPs joined a Commons rebellion led by Johnson in March against Rishi Sunak’s deal with the EU to overhaul Northern Ireland’s post-Brexit trading arrangements.
The question is what does Boris do next? In some ways resigning made sense as a gamble - he could get out of his marginal seat and potentially step into a safe seat vacated by an ally. But if the Tory leadership block him for being a candidate then what does he do? Stand as an independent? Stand for Mayor of London again as is rumoured (which would be a disaster for him as he's not popular in London anymore).
I can't see him just giving up and returning to journalism. Maybe the next leader of the tories will be an ally who will work to bring him back to "save the party" (as the party will probably be a rump party after the next election and has been shaped in his image)
I don't at all see him coming back as Tory leader. He remains popular with Tory members but not the MPs (as has become more apparent the last few days) and certainly not the voting public. Going into an election with Boris as leader would be a recipe for the Tories' destruction. He'll make noises but I see no way he'd get enough Tory MPs to ever put him through to the final two of a leadership election.
After the election, even if Rishi is gone, Boris's route back will be even tougher. First, because much of his support among Tory MPs lies with Red Wall MPs elected in 2019 (who feel more wedded to his brand of 'big state and culture wars' conservatism) - many of whom are definitely losing their seats to Labour at the next election, so won't have a role in choosing the next Tory leader. And second, because many of the southern Tory MPs who may remain - many of whom weren't his biggest fans to begin with but elected him leader in 2019 because they thought he was a winner - are going to feel even more personal animosity to him after the election when his sniping and undermining of Rishi is seen as one of the causes of a potentially huge Tory defeat.
If I had to bet, I'd say we're finally done with Boris Johnson in frontline politics - and good riddance.