This is spot on. As a baby, you typically have two affectionate caretakers who dedicate a significant amount of their time to teaching you language/words, and also shower you with love/praise/attention for every little bit of incremental progress. Also, you're in an immersion program: you often can't communicate your wants/needs until you've learned the language, which adds another layer of incentive.
As an adult, if you were sent to an immersion program in a foreign country with two full-time foreign tutors/caretakers who loved you, like legitimately loved you with all their hearts, you would pick up that new language pretty darn quick.
Exactly; the advantage of input-based approaches is not their speed. You'll learn a lot faster if you spend the time on flashcards, grammar drills, and studying.
The advantage is that it's easier, and often more pleasant, to integrate.
That said, babies do have some neuroplasticity advantages.
We should play to our strengths - babies are hopeless at grammar drills, and adults aren't as good as babies at neuroplasticity - by applying our brain to the problem.
I say that, while having built a comprehensible input tool (https://nuenki.app). But it's useful as a complement to that study, as you can more readily run a browser extension or listen to podcasts than devote your entire life to focused study.
As an adult, if you were sent to an immersion program in a foreign country with two full-time foreign tutors/caretakers who loved you, like legitimately loved you with all their hearts, you would pick up that new language pretty darn quick.