It says that the web API is not yet available for 3rd party developers. Is there an eta for when this is opening up?
Athom doesn’t provide ETA’s on technical availability. You can try and submit a support request here.
FWIW, the Web API is somewhat available for third-party developers already. From within Homey apps, it’s available directly, and there are a few web-based apps (like Homeydash) that use it through a workaround.