I got the background service working despite the broken installer (*) ... but what for?
This extension adds no functionality over the official IDM integrator.
Maybe that used to be different, idk. Haven't used IDM in a couple years and can't remember how things worked back then.

Two stars for at least doing what it says on the tin and not being malware.

(*) for those who still wanna try this thing:
After extracting the files from Github run "install.bat" as stated in the guide, then manually copy all the .js files from the "app" folder to the now created "yourusername\AppData\Local\com.add0n.node folder". Run install.bat again.