The thing is Kimetsu just doesn't have that many volumes to boost. One Piece got to 32kk because it had like 60 volumes at that point. It's not impossible, but it's hard. Each volume would have to be way above 2kk, I think? And considering there's actually a chance the series will end before the year is over...
But well, on the other hand, I doubted Kimetsu sales potential before, and was obviously wrong. Maybe I shouldn't do it again.
Let's say we assume all 3 new kimetsu volumes reach 2mln and the 4th stops at like 1.5mln since it only has 2 weeks or so. That's already 7.5mln, which added to the sales it has right now pushes it to 20.5mln ish.
The 18 volumes currently on the market then "only" have to do about 850k each, which puts them around...2.3mln per volume I guess? Which isn't that much, again if we assume kimetsu can reach 2mln/volume.
Basically if you assume the kimetsu volumes can sell 500k more each before the movie, the movie only has to boost each volume 300k.
Possible? Definitely.
Likely? Well, it depends on how long you think the boost is going to go still. As far as we know the volumes instantly rise as soon as they are restocked and reprints are on the way so it does sound quite plausible to me.
And while yes, OP had a lot more volumes to boost, it was also selling a lot more on its own.