I suspect the recent game update made the Hub manual explanation of a spindown calibration a little out of date:
- Zwift Hub: For Zwift Hubs that are updated to firmware 5.2 or newer, fixed an issue which could cause the spanner/wrench (i.e. calibration) icon to briefly appear on the Pairing screen after pairing the Hub. Manual calibration is no longer applicable because Auto Calibration is in effect.