I see a pop-up when I start typing @ + a username:
The way iNat works (and I assume Discourse is similar) is that it just applies certain settings to any text directly following an @ symbol. The @ mention on iNat generates a notification and links to where the profile with that username would be (if it exists), but that’s it. It doesn’t check to see if the name entered matched an existing username. Discourse at least puts a gray oval under the username if its an account in this forum.