well maybe somebody can type out the beginning of the relevant chapter from hilchot melachim for you.
or you can accept that it says about presuming, and knowing for sure.
Lulab gave a source - hilchot melachim. It's not a big book. I would say , probably chapter 10 is where it is.
But to back up what he said any more than a book name, he'd have to quote a whole chapter. And it's not reasonable for you to just expect somebody to type out a whole chapter.
But since rabbi kahane referred to it a few times, it's worth buying. I have one published by moznaim, but for some reason I can't find it.. I looked earlier.
Anyhow, the fact about presuming and knowing for sure, do not change the demonstrated ridiculousness of this interpretation . which is just wrong on so many levels. Firstly, that it's not even the basis of them saying the L rebbe is the messiah anyway. The basis is either mad speculation, or chabad texts. But not the RAMBAM.
They might try to show that it doesn't contradict the RAMBAM.. Which is still ludricrous. But logically, this is no basis for them proving anything from the RAMBAM. So even if they could show, without being ludricous, that it didn't contradict the RAMBAM, then it's still really not irrelevant.
I can wear underpants on my head, that doesn't contradict the RAMBAM. And it doesn't make a nonsense out of the RAMBAM either! Or make any ludricous interpretation. It just doesn't contradict him. So just having something that doesn't contradict, is not relevant.