Alright, I guess I was wrong above; it certain
is a "USB-C connector port." In fact, I would even say that it
is a "USB port" (although they can't say that for licensing reasons.) Nintendo seem to
expect and encourage you to plug USB-C
peripherals into said port (e.g. any random USB game controller.) Nintendo will support such configurations just fine. You're not voiding your warranty by doing that.
What the port isn't, is a USB certified port. USB certification guarantees that it'll be safe to plug any USB-certified thing into any other USB-certified thing. Without that certification, the device isn't guaranteeing its ability to handle weird things that other USB devices might do—like sending it lots of current without doing a specific proprietary negotiation first.
It's a bit like FCC certification for "accepting radio interference." Devices that have it, are guaranteed to not melt down/throw sparks if you bring them close to e.g. HV power lines, or a radio tower. Devices that don't have it, aren't guaranteed to not do that. They might or they might not; but they weren't required to be tested to find out if they would.
But unlike FCC certification, where it's illegal to sell something in the US containing an antenna if it's not FCC certified, it's entirely legal to produce and market a device that has USB connectors, but isn't USB certified. There's nothing stopping companies from doing it—other than the expectation that consumers might care about the USB logo being on the product. If a company thinks consumers won't care about that in their case, they have no reason to bother.
(That's not to say Nintendo shouldn't have made their product fail safe under out-of-tolerance conditions from other USB devices anyway. It would just be good engineering to do that, even if you don't want to pay the licensing fee. But they didn't think to test for those conditions—likely because the USB Consortium wasn't invited to come breathe down their necks reminding them about things like that.)