This not how SSO should work. What is the point in me creating an account with you and still have the option of signing in with gitlab.com credentials ?!
Mainly to avoid spam influx.
Else, anyone can create a random gmail account, and use SSO to gain access to post spam. Creating a user account makes sure that the account creation request comes from a valid subnet (not an already blocked spamming network), thereby creating less burden on the site moderators. At least that’s my understanding.
Having said that, there are far more stricter policies in few open source domains, (e.g. one needs to send a personal e-mail to the admin with a reason for account creation). In that sense, I guess this solution is way better.