[nilkn]

I think the statement was just phrased too strongly. The proposition isn’t that one shouldn’t study grammar at all, but rather that one shouldn’t front-load grammar and theory too much.

Speaking from experience, I gained basic conversational fluency in Spanish from English in a few months by spending a ton of time with LingQ (essentially reading and listening to untranslated passages and creating digital flash cards), obsessively practicing pronunciation, and listening to lyrics in Spanish music for hours on end. I focused on grammar to the extent that I had to, but no more than that. For instance, I learned only those conjugations I actually needed for simple speech. I compiled the ones I needed to study based on what I actually encountered in real passages.

Now, by no means did that render me fully fluent in the language. But, case in point, I took two years of Spanish instruction in school with lots of grammar front-loaded and I gained more or less no speaking or listening ability at all. For me there was a dramatic difference in efficiency between the two approaches to language learning.