It's probably not worth engaging on a logical level because the people making that argument never reach it. From my perspective, it looks like the argument is a product of retrofitting an existing justification (Kamala is the harm reduction candidate) onto a specific issue (Palestinian genocide) and getting an incoherent result (Palestine will be genocided less). What motivates someone to put that incoherent result out into the world rather than, like, considering it, is probably a product of the shallow thinking social media encourages, an unwillingness to engage with the idea that we don't truly have political agency in the US, and a feeling that there are no other options.

In my less charitable moods I've viewed the argument as an attempted sop where the person advancing the argument does not actually care but does know that if they say as much they'll come across as a monster. So instead they do the bare minimum to retain what they view as the moral high ground in the extremely restricted landscape of the two major parties. Pointing to even higher ground outside that landscape can then be attacked as virtue signaling (anyone can tell that it's unreachable; this is the highest attainable spot) or trickery (anything that looks more complicated than this very simple reasoning must be some form of subterfuge, so I can continue to appear the most reasonable if I just keep hammering on "less genocide"). Absence of pushback from anyone with too much power to be dismissed as a troll or a curmudgeon allows the idea to enter the discourse, at which point other people pick it up and reinforce it.

But it's such a self-evidently weak claim that I can't do anymore than spit ball. It's an argument defeated easily even on its home turf of utilitarianism; any attempts to do that, though, just sends them back to the "Trump is worse" binary.