If you don’t use that username/password combination anywhere, then yes, you are at very low risk.
However, one of the CLASSIC social engineering scams is illustrative. A group made a free porn site. All you had to do was make an account. Free porn!
So millions of people signed up… and a remarkable number signed up with work email addresses. And since the scam was to collect those emails and passwords, the passwords weren’t encrypted as they are on modern, reputable sites. (The encryption is done in such a way that even site administrators can’t read your password.) So the site owners, fresh with their catch of corporate email addresses and a possible password, tried them. Lots of them worked and allowed the hackers to log in to corporate networks and emails.
So, don’t be that person.