Lol dude. Plenty of people who speak English as a second language can speak (and write!) fluently and "naturally" with few errors.
This is confirmation bias. You assume anyone who writes a certain way online is a native speaker, but you have no way of actually knowing that unless they explicitly say so.
I was responding to your claim that you can "usually" tell when English isn't someone's first language. You can't. Unless every comment also states whether English is their native language, there's no way you can know this. It's likely there are a ton of people who aren't native speakers that you don't detect because they write well enough. Plenty of people are fluent in a second language.