Normalising pronouns in bios is a way of protecting trans/NB people, not signalling legitimacy. It's like the easiest way anyone could possibly make lots of unwelcome-feeling people feel a bit more welcome in society.
This goal of "protecting trans/NB people" is accomplished by signalling your support for normalizing trans people indicating their pronouns. If you think that describing it as "signalling" sounds weak or trivial compared to "protecting", it's because it is.
The real protection factor is almost the opposite of sending a signal. It's about trying to drown out the "I am trans" signal that happens when pronouns are listed, by making it blend in with the background. That way someone who finds pronouns important can list them without becoming a big target because of it.
