Found the answer in the parent thread, thank you @Quibblekrust@thelemmy.club:
That’s a mention, not a tag. A tag is a private description you save about a user. Only apps have this fearure.
It’s a little weird that they took a well established term (in social media context: tag, id by which to mention a user, also known as ‘tagging’) and gave it a wholly different meaning (tag: label).
Go tell 'em! Why have alternatives if we can just put all our eggs in one, holey, basket?