It can be done. When the switch is in AP mode you have to go to 192.168.4.1/update then you can upload your own bin file. On my 4 gang it just started clicking so you have to do the device recovery after that.
Cut power from the device completely for 30 seconds
Power the device on and off six times with intervals lower than 10 seconds and leave it on after seventh time
Fast power cycle device recovery should activate and the device should be reset to firmware defaults
Does not support upgrading from homekit to Tasmota
I did it on one of my switches but I don't remember HOW I did it and now I am stuck trying to do the other!