Using personal data to assign a cohort counts as using personal data. Duh. The approach described in the article doesn't use any personal data, though?
Quoting the European commission:
"Personal data is any information that relates to an identified or identifiable living individual. Different pieces of information, which collected together can lead to the identification of a particular person, also constitute personal data."
I'd hazard a guess that it's the second part under which the EC might find this to be within scope.
That doesn't mean you can't use it at all. It just places strong restrictions on what purpodes you can use it for. The important point is just that those restrictions are the same under GDPR for all of these technologies. It doesn't matter how you uniquely identify users, what matters is what you do with that information.
You may be able to look at the headers and see that a certain user made the most requests that day. That still tells you nothing about their identity.
What the technique may fall foul of, though, are cookie laws.