How is trans people existing political?

Gender is a "social construct", such things exist as an emergent phenomenon of society, society does not uniformly and unanimously agree on all of the constructs or aspects thereof, therefore the existence or non-existence of certain aspects, or asserting same to others (such as via the publication of software, or changes to same), is inherently a social and political act.

There are people who think that trans people are simply non-trans people with psychological illness, for example. Regardless of whether or not that is true or false, it is a point of fact that people with those beliefs definitely exist, and that they reject such social constructs as non-ill trans people existing, so asserting that they do (which is what publishing an OS with such emoji in it implicitly does) is a political act (in conflict with the beliefs of those people).

If everyone agreed on all social constructs and narratives, then nothing would be "political". Given that no two people share precisely the same mind and context and social constructs, every action that involves other people or society in some way is political.

Abstractions are leaky.

It's also political because it has specific implications regarding sex-based rights, free versus compelled speech, issues of medical consent - all of which were existing and contentious political issues already.

And also because opposing political groups are using it as a dividing line. Though interestingly, left-wing radical feminists have been consistently the most critical, which is at odds with most others who identify as left-wing politically.

What you linked is the opposite of what you said.

The Unicode Consortium named it "pregnant man". That's the official part.

"Sometimes used in jest" is pretty clearly introducing a usage that is unofficial.