When signing in to Discourse, there is an attempt to link an existing account by email automatically. However, some sites (e.g. Steam) doesn’t give out the users email when signing in.
If a user were to sign in via Discord in one instance, and Steam on another - they would not be given a chance to link their Steam account to an existing account. The only options are to sign up or log in via another method.
The feature request here would be adding a Link to existing account button when the provider does not provide an email address, and Discourse can’t map the new (Steam) account to an existing (Discord) account.