Leaving a comment here for others to find the solution to this issue.
After stumbling upon the actual announcement topic in this forum for the HomeBridge plugin, it is clear from the comments below the announcement that the Konnected boards can only be managed by one instance.
In my case I wanted to test and make a proof of concept, so I programmed it via Konnected Cloud/SmartThings. After that I started the HomeBridge configuration. From the comments it is clear that I first needed to deregister from Konnected Cloud and only program through HomeBridge.
As I have a Konnected Pro Panel this meant the following:
1) Pull out the Ethernet cable
2) Connect a jumper wire between zone 1 and zone 2 on the Konnected Pro Panel
3) Plug in the Ethernet cable —> fast flashing blue light
4) While the board is live, disconnect the jumper wire that is connecting zone 1 and zone 2
5) push the reboot button on the Konnected Pro Panel
This basically resets the board to factory settings. Now do not do anything in the Konnected app, but configure all directly from HomeBridge.
By applying the method above, I have have active sensors in HomeKit through HomeBridge. Keep in mind that you will have to reboot HomeBridge twice after each config change and allow enough time after a reboot for the Konnected Pro Panel to accept the new configuration (goes automatically) and reboot itself (goes automatically).
I now have all the functionality I want:
1) Legacy alarm panel still works
2) Legacy wall keyboard panels still work
3) Sensors statuses are now available in HomeKit through HomeBridge to build further automations (light on when entering the cellar and the motion sensor over there being triggered for example)
4) Monitoring of armed status of my existing system
5) Remote arming of the legacy alarm panel (out 1 on the board going to a relay on the interface panel, triggering a key switch zone on the legacy alarm) when I forgot to arm at the time of leaving the house
Happy with this combination of old tech and new home automation through a relatively affordable Konnected Pro Panel Interface Kit!