You may have meant it like you say but in fact you did write
"..Even people who move to another country as adults and spend 100% of their time speaking that country’s language almost never learn to speak it with native-level intuition and a native-sounding accent (though they can reach pretty high levels). Children do, without even trying"
You simply differed between adults and children. If you meant "it gets more and more difficult the farther you move from childhood".. well fine, but you didn't write the equivalent of 'further north than Brazil' so you should expect to get questioned about that.
No I didn't. I never said anywhere that it's a sharp cutoff rather than a sliding scale.
If I say Canada is further north than Brazil, it doesn't mean that latitude isn't a continuous spectrum.