Learning languages as an adult is a completely different mental process than that of a baby learning to communicate. Most of this article is based on the assumption that all the languages on your head are completely separated from one another so that when communicating in a language you’ll just access the info your brain needs, but that simply isn’t that way.
Normally, when people start learning a second/third language, they take words and expressions from the new languages and translate them to their native language so that they understand what they mean, and that’s regardless of the method you use to learn. That’s simply because adult learners simply cannot learn like a baby. You won’t find an adult who is a native English speaker going to a foreign country like China and expect him to somehow understand things as time goes on without him associating those words he hears to concepts he’s already learned in his native language.
Either way, at the start there’s a connection between languages’ vocabulary, and, as time goes on and you become more proficient in a language or you’re learning your third/fourth language, you instead start associating words to feelings and abstract concepts that have nothing to do with other languages. If you’ve become proficient in at least one more language, I’m sure you’ll have found times where you try to express something in your native tongue but the phrase you’re looking for just doesn’t exist in your native tongue, it only exists in that other language you learned.
This way of thinking that is the one of a more proficient speaker is the one that is the most similar to people learning their native language when they are loud brats. It’s widely accepted between neuroscientists that the way the brain learns and develops abilities and memories is by creating and strengthening new neural paths, while old memories and abilities that you don’t use don’t get strengthened and become irrelevant when compared to the abilities you use. This means that for each word you learn in your native tongue, “there’s a path between the word and the concept it represents”. For other languages, you start having a connection between a word in a foreign language, the closest word in your language, and the final concept. As a result, the word in the foreign language doesn’t correspond to the ideal meaning it would have because it goes through your native tongue in between. For proficient speakers, each word in a foreign language is linked to its own concept, so it your native tongue doesn’t necessarily interfere.
That’s for vocabulary, grammar is a completely different thing. You see, when we want to express something (still talking about vocabulary), we evoke de concept that we want to convey, then the neural paths linked to that concept take us to the closest word (that’s why sometimes you can only evoke certain concepts in a single language). But grammar is all about putting those things together so that they actually make sense, and that isn’t as simple as associating a concept and a word.
Grammar is waaaay more complex than a single piece of vocabulary. It can’t be learned just by reading and hearing people; you need a translation and rules to dictate how to use it, at least when you start learning a language. Sure, you can pick some things up like the general verb position in extremely simple sentences, or where to put the subject, but stuff like pronouns and so on, yeah, good luck learning that from unexplained input.
And here comes the main issue of the article: it assumes that babies learn alone, and that’s not true. Babies hear, babies do, and babies fail, but if they are heard and somewhat understood, their parents, a teacher or someone will correct them. What this means is that “Output (speaking and writing) is less important. It is not the way to improve your language skills.” is a half lie, because if you never speak, you’ll never know whether what you’ve learned is correct.
This is more of a personal experience, but I’ll throw it in regardless: having been born in a region where people start sentences and conversations in Spanish and finish them in Catalan, grammar books and rules are what keeps people here from not being able to be understood in the rest of Spain and speaking proper Catalan. This article underestimates the synergy (or destructive interference) that knowing several languages has, and I say this a someone who speaks two native languages, a third to a decent level, a fourth to some extent and just a bit of a fifth, yet due to learning more languages, I can’t even speak my own native languages the same way as someone who only speaks Spanish natively.
So all in all, yeah, pretty much what Helios said: immerse yourself in the new language and live in a foreign country if you can, but you should still be looking for lessons or a grammar book if you wish to survive the first year and have a strong foundation to build your new language upon.