Nintendo made an informed decision and their hardware is welcome on my networks.
If you assume basic _routing_ doesn't work, why do you assume a TCP connection from one to another would work? If an AP implements client isolation (as many home models do now for "guest" networks) broadcast would be the only thing that works.
Pretty much every printer you buy now uses Bonjour for printing, so your average SOHO router is going to make sure that works.
I can't fault them, at one point I thought this was the right way to do autodiscovery on a LAN too. Then I learned from my mistakes.
Bottom line is that nothing on an IP network should do something st00pid if it gets a random weird packet from a random IP, because the whole history of IP shows that you will, eventually, get one.
Edit to add: and Pokémon games have used peer to peer protocols before on different systems, so this use isn’t new to Pokémon or Nintendo systems.