It means deleting your Facebook account will also prevent you logging into those other services. I had to delete a Spotify account and start over after I initially signed up via a Facebook login.
(I should note that the Spotify staff were amazingly helpful and actually migrated me over to an email-login account when asked.)
SyneRyder's post (https://news.ycombinator.com/item?id=11569891) was specifically adduced to demonstrate the dangers of logging in with Facebook, and mentions that doing so was an accident, so explaining the dangers and suggesting that he or she not do it again seems patronising.
