Educated guess/experience, I don’t have anything concrete to back it up. With ChromeOS I’ve had this same trouble with other apps (music production, for example) designed to remote control a program running on a computer on the same network. They just never find the app on the network, which suggests this is a networking restriction or limitation in ChromeOS.
If you are referring to Android apps, I believe they are effectively on an internal 10.x.x.x NAT network hidden behind whatever network the device is actually connected to. However I don’t see why Companion would need to broadcast anything on the local network. Zwift obviously operates on a client/server model where they run the central servers, so it should be possible to coordinate everything through those.