Facebook and Spotify are tightly partnered together - at one point they actually REQUIRED the use of Facebook to log in. The option to register without Facebook was only reintroduced recently.
For Facebook they do. They have a tight partnership. If Spotify did something wrong with their Facebook app, Facebook would have removed their app a long time ago.
In the US, when spotify first opened up to Americans you could sign up without facebook credentials. About a month later, they decided to go facebook only for new setups.
Spotify started requiring a Facebook login from all new users on 22nd September 2011. On 6th December 2012 they put the non-FB signup option back. You signed up before they required FB so you never saw the requirement.
If you read the reply again, they allowed you to use a username, then required facebook, then reintroduced the ability to user a username. You probably were using a username, before they started requiring facebook.
