Especially since there are plenty of people who aren't native speakers, but got to the native level of fluency (usually, due to moving to an english-speaking country at some point and spending 99% of their time in english-only environments for many years). How do we account for those?

I always had trouble with the whole "native speaker" categorization, because I definitley don't fall under it by definition, but English is legitimately my most comfortable and most frequently used language by far, and has been this way for over a decade.

Like, the only thing that gives away my non-native status is that sometimes my accent comes out accidentally, but that's just stupid muscle memory. I am way more comfortable with english in every aspect (speaking, writing, etc.) than my actual native language, despite often having an accent in english, but never in my native language (which i have exactly zero foreign accent in).