[hyperman1]

I'll try an analogy. Your router is an apartment building. The switch is one of the many tenants. The problem: Mail for the switch does not arrive in the correct pidgeon hole of the building.

Problem one: Nintendo wants you to take your IP and add 20. So if you live in blahstreet 1 box 7, they tell you to send all Switch post to blahstreet 1 box 27. If someone else happens to own that box, bad luck for him. Worse, the IPs are dynamic via DHCP, so a box unused today may be used tomorrow. But that's the minor part.

Problem 2: Nintendo wants you to tape a big paper above the pidgeon holes, saying: Postmaster, please drop all mail in box 27, no matter what box it specifies. Technically only for UDP, so the TCP traffic will be delivered. Even so, a lot of post will be wrongly delivered to the Switch and thrown away. Other tenants will wonder why they don't get their mail.

Problem 3: You become very vulnerable. The router will send every hacking attempt to the Switch. The tiniest bug in its networking is now critical, as outsiders can just assume your Switch will receive anything they send.

To recap, Nintendo is making sure their device receives traffic by throwing every other device you own under the bus. Even a second Switch will suffer. The advice is Dilbert PHB level idiotic but will seem to work for a while.