But no one does deny that "trans" exists, in the sense of denying there are people who believe themsleves to be "trans". That is a strawman.
They simply disagree with what that means.
Is it "real" in the sense of an actual occurance of some mental element that is usually core to the self-identity of one sex occuring in the other? If so, that fundamentally changes the basis on which all of us understand ourselves. I think that isn't someting we can just hand wave past without interogating what it means for our understanidng of what is it to be a man or a woman at all.
Is it "real" in the sense of something like colourblindness, where something is wrong with the body's perception systems such that the individual's perception is exactly the experience a person without that broken sense would have were the world to actually be like that, but in reality the world is not like that, it's a faulty signal in the system. What the individual is experiencing the world to be is to them exactly the same experience as if it truly were, but this this does not align with reality as perceived by everyone else?
Is it "real" in the sense of a delusion akin to those experiened in psychosis - this time not a faulty signal about reality from the perception system to the mind but a flawed processing of the correct signals of reality in the mind?
Or is it "real" like a religion or a secular ideology is real, whereby individuals find or are brought up within a belief system that makes sense to explain their world experiences and deeply believe it to be true, but it is ultimately based on an external belief framework in which one places ones experiences rather than any "real" experience of the material or mental world?
Whether one or none are or all are actually true, these are all things that could exist in the world.
But actually being/becoming the opposite physical sex either through medical intevention or some metaphysical "body of Christ" sleight of hand? No.
So when people say "trans doesn't exist" - they aren't denying the exietnce of people who claim, many entirely genuinely, that they experience themselves as trans. They are simply saying since there is no basis on which one sex can physically transition to being the other sex, whatever the experience of "trans" is, because it is not actually becoming or in some metaphysical way being the opposite physical sex it doesn't change anything in our shared reality about the person's actual sex, their sex as others experience it, so we should not be treating it as if it does by opening up the language, protections and rights of physical sex to accomodate these "trans" identities.
Existing as the opposite sex in ones heads, even if due to "real" causes, is not the same as existing that way in the experiences of other people as well.