If you gave me this ID number, I could use it to locate your information in breached db dump, or if it is used in API requests, impersonate you.
You're suggesting it's an authorization token - which it obviously is not.
I doubt this vulnerability exists, but these IDs (and any IDs by any company) should be guarded just like any other PII for exactly this sort of reason.
IP counts as metadata. It uniquely identifies you as an entity but does not reveal other details except geographic location. If IP addresses are PII, then any use of the internet is violating your privacy. Perhaps unplug your modem, turn off cell service on all devices and read a book instead.
Knowing an IP address can distinguish user A from user B, but unless you know something else about A vs. B, what's the point?
Knowing an IP address is useless information, until you have a database linking IP addresses to geolocation. Knowing my address is useless information, until you have a map. Knowing my name is useless, until you have Google. Knowing my user id is useless, until you have a leaked database (or other vulnerability).
These are all PII, because they're useless until you have some other information, and then they deanonymize you.
I will reiterate my point. It is impossible to operate the internet or any other network where a server must distinguish between two or more client without some kind of identifier for session management. Just think about it.
The GDPR never mentions "personally identifiable data" as that is a US term. In the GDPR, it only says "personal data" which is the exact same thing according to the GDPR.