Forget all the stuff out there that says the GDPR protects EU citizens. This is a question of jurisdiction and enforcement. Say I run a blog under a business registered in the US funded by advertisers in the US. A EU citizen that comments on posts issues a GDPR request that I ignore. Their government fines me. I tell them to get bent, I am out of their jurisdiction. What can they do at that point?
I agree about logging IP addresses or emails.
But I am not so sure that usernames or nicknames are necessarily identifiers. For example, if someone posts as “IamtherealTomHanks”, you can’t actually identify who they are.