It says you can connect it to a Garmin Forerunner 245 and then rebroadcast to Zwift. Does it still work for recording treadmill stuff with just the Forerunner i.e. if you’re not Zwifting?

Yes, but accuracy will be a bit hit and miss. You can calibrate it but since treadmills don’t really reflect running outdoors it’s a bit pointless.

Your 245 will pick up cadence and distance from your wrist anyway, it’s not like the Zwift pod will give more data.

Stryd pods are accurate but expensive.

But if I’m not moving, how will Garmin know how fast I’m going? This is for my crazy walking desk experiment anyway so I expect my wrists will be stationary. Just looking at a rough estimate and I’d rather not spend £100 on a Garmin pod.

Oh, right. If your wrists aren’t moving then this is the next best thing. But your watch is expecting you to run so it’s going to be wildly off for distance. Personally I’d just go for a £2 pedometer and count steps.