That babies can learn multiple languages with ease is a surprise to no one in the world.
Only surprise is that education systems around the world continue to waste so many years of people's lives trying to teach them additional languages when they're teenagers, instead of having done the bulk of the work when they were babies, the way that nature's been doing it successfully for tens of thousands (or more) years.
This is actually a somewhat controversial claim, academically. There's pretty strong evidence for a "critical period" for acquiring a native-sounding accent, and some evidence that certain adult learners still occasionally mess up minor grammatical details after decades of immersion. But other studies suggest that language-learning ability trails off gradually with age in much the same way that, say, physical ability does. A 35-year-old can still get in pretty good shape if they work at it.
I know plenty of people whose language-learning strategy is basically, "Put myself in situations where I need to use the language and I can puzzle things out. If I get curious, maybe look something up once in a while." It's not quite like childhood acquisition, but neither is it a completely different process.
"Provably" how? Many people learn through a class and go on to speak a second language, so what's the proof?
Sure, eventually you just have to use it. But as an adult learner, you can get going a lot faster by using your knowledge of other languages, understanding of rules, and so on, before jumping in, and it's not always practical for people to just take off and spend months or years in a foreign country in the hopes they'll learn a language while they're there.
If you're just saying that they're not perfectly identical, then that's obvious, you've said nothing. What I'm replying to is your implication that they're vastly different. Science shows that they're not as vastly different as the education system assumes.
Only surprise is that education systems around the world continue to waste so many years of people's lives trying to teach them additional languages when they're teenagers, instead of having done the bulk of the work when they were babies, the way that nature's been doing it successfully for tens of thousands (or more) years.