thank you for your comments! I've followed you suggestion to watch the videos - and learned a lot! With a switch node I was able to filter messages from the tradfri-monitor node based on the internal instanceIDs of the light bulb, so thank you for pointing me in the right direction. Now I can use the Tradfri remote control, buttons or the IKEA app to change the light and forward that change to my Loxone home automation system and vice versa.
I still don't understand the usage of the tradfri-state node, but maybe I have to get deeper into node red first.
With the current approach I have to sniffer the instanceIDs via debug node that is connected to the tradfri-monitor node and add the specific ID of my light bulb to the switch node. The final output is a tradfi-light-control node where the specific light bulb (having that specific instanceID) is selected by a comfortable drop down menu and the user friendly name. Maybe that's due to the fact that the library is in an early development state? Maybe the IKEA tradfri gateway is able to send any changes to a "hook" that is implemented by the tradfri-monitor node (without polling)?
I'm also for more documentation about the way the IKEA tradfri gateway communicates withe the app. I found some good starting point at GitHub - glenndehaan/ikea-tradfri-coap-docs: How can you communicate to your ikea tradfri gateway/hub through coap-client with the coap-client to read stuff from the gateway, but I'm not sure if polling is the only way. At least the tradfri-monitor is sending any changes from IKEA remote controls quite fast.
Still I'm wondering if there are any people out there who use the IKEA Tradfri gateway instead of a zigbee gateway (either Philips Hue bridge or USB-Stick like conBee)? I've found a lot of examples for the others, but no examples for this library or the @fetchbot/node-red-contrib-ikea-home-smart library.
I like the IKEA tradfri gateway, the secure coap communication and the "local only" approach. This fits perfectly to my Loxone home automation system. It's a bit sad that IKEA has not published an official documentation about the API they use, but it looks that all important facts have been discovered already.