A natural person is a legal term so it's really not a matter of opinion... Believe it or not, usernames only become PII if the user choose to use their real name or another information which uniquely identifies them in real life
I can understand your point and it was nice to entertain this conversation with you. I can't see how to convince you from my view and I don't see how you could convince me of yours. But I will keep you arguments in mind and will look for some more information regarding to it. Thank you!
GDPR handles all kinds of data related to a person, so yes: a username is personal data wrt GDPR
personally identifiable information (PII) on the other hand is only a subset of data handled by the general data protection regulation (GDPR)
This might clear that up https://techgdpr.com/blog/difference-between-pii-and-persona...