You would also want a dongle that has a driver already bundled with the OS. Not sure if that's typical or not. Probably unlikely that the dongle itself comes with a suitable ARM/Linux driver.
USB audio devices are standardized (just like flash drives, as USB mass storage, and keyboard/mouse, as HID respectively), so you don't need device-specific drivers for most of them.
Oh. Either that post was edited or I misunderstood - I thought they were talking about an HDMI dongle with 3.5mm audio jack for audio extracted from the hdmi. But yes, indeed, a USB audio dongle should "just work".
I don't think HDMI audio extractors need a driver, either. The first one I could find claims compatibility with all kinds of source devices. It's probably entirely invisible to the source. I don't how know about HDMI DRM, but that's not a problem for the RPi.
They don't, but they can be finicky. Most can only extract audio if it's specifically sent as stereo, if the source sends audio as encoded 5.1 stream then most don't know how to decode that. So you will need support for being able to chose the output format over hdmi, which is where I imagine the difficulty in the RPi will start.
I add my voice to the chorus of complaints about dongles, but for the record you could also use an inline HDMI audio extractor which is invisible to the OS and requires no drivers for sure. They are also very cheap.