This is absurd and 100% bullshit. It means anything and everything is now personal data, even randomly generated data.
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.
Isn't the purpose of global rankings to know how any given user is ranked globally? It's not much of a ranking system if all you can say is, "Yes, we have one million users, and they are ranked 1 to 1,000,000. We are not able to identify who is what rank."
>Furthermore, in such situations there's a case to be made that this information cannot be deleted if the whole point of the site is to provide global rankings.
You don't have to delete it, anonymizing is enough. For global rankings you don't need to know that user 1456 has rank 56, you just need to know that somebody holds that rank. So whenever a user requests to be deleted, replace his id with -1 (or choose a random number outside the normal range) and you have satisfied the request (only the PII needs to be deleted, not everything about the user).