I remember when Mullvad offered port forwards, it assigned you both a port and a key. My guess is that they simply authenticated the key to determine if the port should be routed to whatever tunnel established the connection. So, they would have to map that a dynamically generated key was assoicated to the port (and probably other bits of non-PII like datacenter/region), but nothing beyond that point outside memory. Even account IDs they generated were dynamic. In theory if you were able to guess the ID, you could use the account.