I’ve had the emails about this earlier and haven’t looked into these but would I be right in saying this is their equivalent of an Arduino?
Pretty much yes. It’s a microcontroller rather than a full blown computer, so is nowhere near as capable as a pi zero for example, but conversely will use much less power.
Some guy used one of those controllers to add a rotary volume button to his touch-screen radio (that didn’t have a physical button).
At $4 this looks like reasonable project… The rotary encoder costs more.