as far as i see it is nothing other than Firebase Messaging.
Ok, a bit greater payload possible.
You app need to run on a device to get his id.
It is the same for Firebase messaging. If you want to send to a specific device you need to have the id of this device.
The example here does nothing more. It register the device to the server-component (a python script) to later send a message to this device.
You dont need a SERVER running for firebasemessaging. A simple Hosting will work to store the usertokens...
Yes! BUT you cannot show notifications on Amazon Fire Devices with firebase. my app - has a slight modification to make it Amazon Fire friendly. its the same app on playstore and amazon. it has a lot of installs on Amazon about 600,000 but only about 160,000 on Playstore.
If i send a firebase message it only shows up on Google clients, not Amazon. Leaving 600,000 people without a notification. (and missing ad revenue

)
---- EDIT - Unless the message is being received but not displaying?