looking at the code that seems to be the id of the website not of the user. You might have multiple websites on one instance of Counter and you just need to let the server know on which one to record the view.
IIRC, GDPR doesn't really care about cookies, but about storing information on the user's machine. sessionStorage would still fall under this classification. So it doesn't really matter if they are different or not.
That said, it does seem like Counter does not use user ids.
