Thanks Le Baron and lysi for the explanations! I also found this sentence from the Wikipedia page on
Input hypothesis: "Understanding spoken and written language input is seen as the only mechanism that results in the increase of underlying linguistic competence, and language output is not seen as having any effect on learners' ability." It isn't sourced but I'll assume that it's accurate, and apologies if it isn't.
Now again, I do not have a PhD in linguistics or education so I can't provide an academic refutation of this, but I have to say that it makes no sense to me and goes against everything I have experienced as a learner.
Shut down the conservatories. Want to be an opera singer or concert pianist? Don't practice your scales, just listen to a ton of music and you'll be ready for the big time! Want to be an NBA player? Don't bother practicing lay-ups, just get a season pass on ESPN! Want to be a chess grandmaster? Don't bother learning opening theory or solving puzzles, just watch Twitch streams all day!
I am a classically trained singer and I have to say that producing the sounds of a new language requires just as much training, drilling, and muscle memory as any other task like learning to sing or play piano. I work very hard on my pronunciation and I've had very good responses to it. Icelanders have told me they were shocked that a foreigner could correctly pronounce words like Gull or Eyjafjallajökull. On my first Xhosa lesson, out of dozens of different sounds my teacher only corrected me on one. The French are notorious for switching to English when they hear an anglophone accent, but I have never had that happen and I don't recall a French speaker ever misunderstanding anything I've said or asking me to repeat myself, let alone switching to English. Do I have some sort of special magical throat? Nope. Am I making it all up for internet attention? I'd say no but that's just what someone who's making it all up for internet attention would say so you'll need to trust me.
The truth of the matter is, my success with pronunciation comes because I drill sounds over and over until I can consistently get them to sound right. When I did Pimsleur Icelandic I didn't just respond once to each sentence; if something didn't sound exactly right I'd repeat it over and over until it did, and maybe later in the day I'd still be muttering some particular problem words. I had been practicing Xhosa sounds, especially the clicks, for weeks before my lesson, and it was not my first time trying to wrap my head (and tongue) around it. I've also done my best to practice my French pronunciation as well as I could, and I think it's good now but I know I have a lot of improvements left to make before I'm truly confident in my pronunciation.
In the case of Icelandic and Xhosa, I did not spend hundreds or thousands of hours listening to native materials, and in fact I have not listened to much at all. Instead I focused on specific sounds and practiced them over and over until I had something that sounded correct in my muscle memory. In the case of French, I have had a lot of input and I can read it pretty fluently, but I can't speak it nearly as confidently as I can speak German, even though my German vocabulary isn't nearly as strong and I haven't had as much input. I think the reason is that since German is more closely related to my native English, it's a bit more comfortable to pronounce so I don't need to make as drastic of a change from my native accent.
Another counterpoint is that there are many people who have had thousands of hours of comprehensible English input who do not speak perfect English. They may have fossilized grammatical mistakes, they may not be able to distinguish between important minimal pairs, or they might not have the confidence to say much at all, and yet they can understand everything an English speaker says perfectly. They are already way past the point of diminishing returns on input and no further amount will do them any good. I think that if you truly want to get your pronunciation as close to perfect as possible, you need to drill the correct sounds (hearing and producing them) and get them into your muscle memory, just like you would do if you were learning an instrument.