> It matters little what subset of USB any given device supports. No reasonable person will expect, say, high-speed video from a cheap mp3 player.
No, it's both. Laptops ship that have one USB-C port that will carry DisplayPort alternate mode, and another, equivalent looking port, that won't, because it's not wired to the GPU. Consult a manual to figure out which port or ports is wired for video.
Your Nintendo Switch will work with an DisplayPort demuxing dock to output an HDMI signal, but put the forthcoming Nintendo Switch Mini, with an identical looking port in the same dock, and no video will output, because the mini lacks a DisplayPort crossbar mux IC connected to the port, and only uses it for charging and USB-C peripherals. This is a frequent source of complaint on gaming forums.
Laptops ship that will charge off of one USB-C port but not another, because the manufacturer saved a few cents by only wiring one for the optional high-speed charging spec.
Etc. Etc. Etc. While some things obviously don't support some USB 3 or USB-C optional features (obviously my USB-C keyboard does not output DisplayPort), consumers are often frustrated in other cases when their intuition and the equivalent looking ports suggests something should work when it doesn't.