Why? You'd have to ask the authors why they put up a transphobic site. My guess is that they're either personally hateful bigots or just amoral cowards who take money from the same.

As for how...

[How] is it transphobic?

Go read the "about us" link on that advocacy page you keep pushing. Specifically the part about how "activists" are trying to confuse their "truth".

The advocates they're talking about are non-cis LGBTQIA+ folk whose dastardly agenda is "just let us be who we are." I suspect that if you scratched down to their funding source you'd find some of the same people pushing bathroom bills and gender marker shenanigans on official documents.

[How] is it an oversimplification?

For the Nth time, it's an oversimplification because categorizing all humans based on which reproductive cells they make ignores or misgroups humans who either don't produce any or who look and smell like they might produce the other ones than those they might once have produced.