Would it? Not sure if that’s necessarily true. They can’t replace him on the ballot either way so they’d have to force a write-in campaign.
If Moore publicly announced he’d step down, people could vote for Moore knowing he isn’t the one who would actually serve the Senate term. You don’t have to do a write-in campaign, just go with Moore and a pledge to step down once elected, and you get a Senator picked by the Republican governor. That’s actually the move with the best chance of winning at this point IMO. You get your deplorables voting for Moore because they actually like him, and your sane Republicans voting for him because they like the governor. And that’s the coalition that delivered Alabama to Trump both in the primaries and the general, with hefty margins.
Could Moore break such a pledge? Absolutely. But the same people who got duped into the “Trump will pivot to the general/pivot to acting presidential” will easily fall for it whether it’s an honest pledge or not. And the kicker is, I don’t think McConnell cares either. He needs the pledge to win the seat. Once he has the seat, it only matters if they have an (R) next to their name. Moore could break the pledge or not, it won’t matter.