I once did an English language vocabulary test that yielded that I'm amongst the top 0.01% in terms of amount of English-language vocabulary.
English is not my mother tongue and I still and often make mistakes in the use of "in"-vs-"on" or even in certain forms of past tense.
However I read a lot in English, in various areas of knowledge, plus it turns out lots of really obscure words in English are pretty much the same as a the word in some other language I know or even pretty much the Latin word, so when I didn't know that was the English word for that, I can often guess the meaning.
All this to say that I absolutelly agree with you that it's a reading thing, plus at more specialized language level, the "knowledge of foreign languages" also has some impact.