It seems to me that the author is ignoring the relationship between experience and expertise in a language. Programmers call themselves [language/framework] dev

It seems to me that the author is ignoring the relationship between experience and expertise in a language. Programmers call themselves [language/framework] developers because they've invested a lot of time becoming proficient in that language/framework. Of course there's some skills transference when you pick up another language, but they're not commodities and they're certainly not entirely fungible.