Why is saying a transman or transwoman is not a man or woman considered transphobic?

I thought “transphobic” would be more like believing transgendering people deserve to be discriminated against or not deserving of basic human rights, or deserving to die because they’re delusional freaks (not my wording), etc. However, apparently, that’s not the case. What’s actually referred to as “transphobic” is all of the above, PLUS people saying a transgendering man is not a man and a transgendering woman is not a woman

What gives?
Why is saying a transman or transwoman is not a man or woman considered transphobic?
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