I agree, it does seem outdated(though the question also mentions USUM) but it's the official developer portal so what can you do.. and I agree, Game Freak does very likely use C++ or a proprietary engine and/or scripting language.
But if it's about learning to make games I still think Unity is great starting point.
And just a related warning to anyone looking to develop for a nintendo system, please take their NDA seriously.