That's not a WebFinger URL. WebFinger URLs point to /.well-known/ path that returns a JSON document with links to other places for the user. To simplify WebFing

That's not a WebFinger URL. WebFinger URLs point to /.well-known/ path that returns a JSON document with links to other places for the user. To simplify WebFinger use by a human, we introduced acct:user@example.com syntax. And (at least my) WebFinger clients can assume the acct: part, much like email clients do for the email: scheme.