This is not a gotcha, I don't know why people think this is a gotcha. RFK was assigned male at birth. The anti trans people are not exhibiting hypocrisy here, because their view is simply (if translated into the correct parlance): "People should not receive hormone replacement therapy that is contrary to the gender they were assigned at birth".
It's not a cool view of them to have, but there's no hypocrisy there, especially not to them. So when your far right trans fearing relative says they hate HRT, and you say "but RFK!1!1!1", don't expect it to go anywhere, not because they're an unreasonable jerk (although they probably are that, too) - but because it's legitimately not a good argument, because it does not exemplify any contradiction in their beliefs.
This entire piece of news being presented this way is nothing but choir-preaching, and displays a serious misunderstanding and mischaracterization of the other side. The debate will never move anywhere if we remain too proud to understand our opponent's arguments and instead delight in tearing down strawmen.