Earlier in this thread I mentioned someone made an integration of the SIA protocol in Home Assistant. So someone else forked this project and owns an AJAX alarm system: https://github.com/eavanvalkenburg/sia
The installation was quick and simple in Home Assistant. After that I made sure HA made URL calls to Homey when the alarm is on or off. In Homey I created a virtual alarm device to react on these calls. I now have a working on and off state in Homey. Very basic and simple, but at least I can now integrate the arm and disarm states of AJAX in my flows.