This seems like a bad shibboleth.

It’s actually easier for astroturfers to pick non-autogenerated names when they create accounts because they won’t feel any pressure to put much thought into it.

I would also bet that the vast majority of human users don’t bother to pick their own name because they don’t initially intend to post, they’re just creating an account because that’s all but required just to even read Reddit these days. Some fraction of them might later be motivated to comment, but by that time the account already exists.