Hey all -
The Zwift Hub trainer will autocalibrate as long as it’s on firmware version 4.x or newer. Currently - latest firmware is version 5.2.2. Please see this support article for the details if you want to force the calibration to happen.
One thing to note is that your Hub trainer must be paired over Bluetooth for the calibration. It will not calibrate over ANT+.
This is to say there is no longer a need to manually calibrate, and the wrench icon should not be displaying at all. This will be fixed in the next Zwift game update v 1.52.