It's absolutely a username. It can be changed arbitrarily whenever you like, and you'll probably in the future be able to have more than one name for the same underlying account, but it's still a username.
Other services do this too. For instance, you can sign up for some services with an email, and that's what you use to sign in, and you might be able to find other people by email if they let you, but you don't necessarily get shown someone's email on their profile, just the display name in their profile. And (in a well-designed service) you can change your email address at any time.
Other services do this too. For instance, you can sign up for some services with an email, and that's what you use to sign in, and you might be able to find other people by email if they let you, but you don't necessarily get shown someone's email on their profile, just the display name in their profile. And (in a well-designed service) you can change your email address at any time.