This is what I think is happening.

Pressing "Sign in with LinkedIn" is deceitfully vague, and will create a new account if you don't have one already. So I actually created an account and then immediately deleted it afterwards, but their welcome email was delayed for whatever reason.

Just to be sure, I pressed the "Sign in with LinkedIn" button again and it made me an account again. I waited for the welcome email to come in, and then deleted it once more. I hope that's enough.