Its not just the gesture, its everything else as well. You know, repeating nazi conspiracy theories about jewish people wanting to eliminate """whiteness""", supporting the AfD, etc.
And I think that yes, he would do that. I mean, look at how many people are sanewashing this and trying to pretend he hasn't even done it. Look at the fact he was cheered on by the people in the crowd while doing it.
America (or at least enough parts of it with political attentiveness and less apathy) is so far right that advocating for what amounts to mass ethnic cleansing/purges (as Trump/GOP has done, and as fascists do) is a completely accepted position by at least 30% of voters. Not to mention the plans to suborn the entire US government into a hyper authoritarian, totalising christian nationalist social-regimentation-enforcer, amongst so many other things I can't even count.
I'm trans nonbinary, and I get very strongly bothered (not in this case as you are asking but...) when people suggest that being nonbinary means you aren't trans (or that they are mutually exclusive groups), as my gender is distinct from that which was assigned to me at birth nya.
I share experiences with a lot of other trans people as well and so having people imply that I am separate from that community bothers me.
This combines with the fact that such separation often implies that nonbinary people want or need only a subset of things as binary trans people may often want/need (such as various aspects of medical transition), which is untrue - for example, I have HRT, much like many (but not all) binary trans people want or have, but often people separating "nonbinary" from "trans" seem to operate with the implicit subtext that nonbinary people do not transition or do not transition in ways they consider as "real" or "full" or "as important" as binary transition (not that I am suggesting that is actually a thing, just illustrating the subtextual framing I perceive - its also worth remembering that binary transition pathways/goals/etc. vary a lot between people too).
Not only this but this separation often comes with the implicit subtext/assumption that nonbinary identities are not strongly separated from their assigned-at-birth gender identity. There are certainly nonbinary people like that, but there are also lots of us who have an identity quite far from that assigned at birth ^.^.
Some nonbinary people don't consider themselves trans, but a lot of us do (including me).
In summary, for me this is because my gender identity (strong sense of gender but not one of the "man/woman" genders and instead a third axis with some fem component) is strongly distinct from my AGAB, and I do/want-to-do things that other people in the trans community often-but-not-always do or want to do (such as accessing HRT, legal name change, surgeries (though not conventional binary ones), etc.)
As for the etymology, just think of it as cis=same, trans=different - rather than cis=same, trans=opposite.