A lot of user ids are literally randomly generated data (for example any UUID used as user id). That doesn't make it less useful.
But that doesn't mean that everything is personally identifying information. "I am 53 and like Harry Potter" isn't enough to identify me, so it isn't PII and isn't covered by GDPR. My Blizzard username is enough for Blizzard to identify me, so it is PII.
The important concept here is that in Europe, data protection laws are not only protection against corperations, they are also protection against nation states. We had a few bad experiences with the effects of government surveilance, and we had a few bad dictators in the last century.
This is absurd and 100% bullshit. It means anything and everything is now personal data, even randomly generated data.