[Quoth]

All WiFi interfaces provide WiFi. The same client works with any of them. It's not a valid comparison. Also most routers with WiFi can be set to just be Air Points, some can be relays (Mesh) instead. A tethered phone can sometimes be a WIFi hot spot. The same laptop, tablet, phone or ereader with same settings will work with any of those WiFi devices.

Accessory mode is just an analogue connection using the USB connector instead of a 3.5mm or 2.5mm or 1/4" jack. USB Audio is always a data mode, digital.

Analogue uses a direct varying voltage and the driven device can be purely electro-mechanical. Doesn't need any electronics or software.

Digital on USB uses serial digital data and a protocol stack. The data can mean different things. You can even interleave the data for different applications on the one digital serial transmission. USB Audio is one of many purely digital serial data modes on USB.