I think we have to distinguish between "learning a language" and "picking up a language". In every single immigrant family I've ever heard of (including mine) the children "pick up" the language faster than the parents do, even when they try to "learn" it through conventional/classroom methods. It's almost a law of physics. The example you cite just goes to show that children don't "learn" a foreign language in a vacuum any more easily than adults.
A Fixed viewpoint will say that children's brains must be wired for learning languages, and adults have no chance of emulating that so we're stuck with what we've got. But the Agile viewpoint will say, "What are children doing right and adults doing wrong?"
Mnemonics and other tricks are only good for language as knowledge ("learning"), all they do is help the element in question last longer than its usual short-term memory shelf life so it has a better chance of still being available when needed in a real-life situation. After a few of those the word becomes part of a language skill ("picking up") and then the mnemonic or other crutch is no longer needed.
Part of what gets me so excited about WAYK is that instead of coming up with just another "learning" crutch it sets out to help adults "pick up" language straight off the bat. As a result, whether by coincidence or design, many of its elements ditch "adult" assumptions and prejudices in favour of "childish" approaches, thus reflecting an Agile mentality.
For example, it takes TPR (which observes that small children tend to move around when they learn language, and assumes this is relevant) and takes it to the next level by combining it with sign language.
Then there's the "fun" element. WAYK makes a language learning a game and not a class, going back to something children do automatically.
It rejects stress and goal-oriented pressure, again, the mistake of adults. The carrot of success is preserved without the stick of failure, eg. How Fascinating.
Instead of giving students head knowledge about one language by "digitally" encoding it in another, it mimics real-life situations where language is actually needed, thus providing more natural, "analogue" information at the same time as replicating a scenario where that information is attached to words. Insert "fairies" talk here.
In its ideal form it's even better than immersion. Immersion leaves you to "pick up" the language by chance in the course of random situations where you need it. WAYK, on the other hand, aspires to take all those disparate learning moments and package them together.
So I think it's not wrong to start by saying, "Who learns language better than anyone? Children", because in practice this is true as things stand. But as you say, this must quickly be followed up with, "But with the right methods adults can actually learn language better than children. Its not because children are super smart, it's because adults are doing it wrong."
We could continue with "WAYK seeks to rectify that. I'm telling you this because I know you're going to find some of these methods silly, embarrassing or even childish. But sometimes this is precisely why they work. So don't worry, we're all in this together. Forget about stressing and concentrating and memorising. Just let your hair down and enjoy the game."