Except that gender is a psychological and cultural phenomenon, so believing in something different in this case, by definition, means that this different state exists. It's like saying that if someone describes a feeling that you haven't heard of and you claim that that feeling must not be real. It doesn't make sense.

Besides that, like racialism before it, the Western binary division between male/female sexes is also seriously outdated and doesn't reflect modern science, which posits the existence of a spectrum of sexes, so even if you pretend that cultural/psychological phenomena like gender shouldn't be taken seriously (which is dumb) you still run into the problem of sex not being binary either.

Just as hetero science completely missed the reality of non-reproductive sex and homosexuality in the animal world, so too does cis-led science tend to try to frame things in simplistic, binary, cis models even when it's clear those models are painfully outdated... but of course someone who jumps straight to "those natives and their human sacrifices" probably isn't thinking that critically about how their own culture's biases affect our understanding of the world as well...