There is a misrepresentation of what "deny our existence" or "debate our existence" means in terms of the word existence.

Obviously we physically exist. We're not going to disappear into a puff of air just because some transphobe says they don't think being trans is real.

But existence doesn't just cover the physical. If I say that Judaism isn't real, that erases the idea of Jewish people from existence. This would be heinously wrong to do, which is why we don't do it.

A common trick for transphobes is to try and frame our lives as being pretend, made up, or based on feelings. This is another aspect of what we mean by "debate our existence". Transphobes do not get to say whether or not trans people are really trans, whether a trans woman is really a woman, etc. That debate is not available.