The article is about bilingualism, which here means speaking two languages natively. This is vastly different from speaking languages you've learnt later on, a

The article is about bilingualism, which here means speaking two languages natively. This is vastly different from speaking languages you've learnt later on, and only partially master (or even speak very well for that matter, as is often the case for people who have learnt English). I speak several languages with wildly varying degrees of proficiency, but only speak two natively. I can confirm that switching between my two native languages is seamless, and I usually don't even realize I have switched.