Upstate1618 Apps choose which of the data they have available local should be sent to their own services or other services. They choose how they protect that data. As an example, Signal doesn't send any data through FCM but rather only uses it to wake itself. Any other E2EE messaging apps do the same thing or send the data through FCM end-to-end encrypted. They don't have the non-E2EE data so they cannot send it through FCM. They can E2EE the whole FCM message to avoid giving more metadata to it than the time, destination and size of the message. The only reason to send data through it is avoiding a connection to their own server to display the notifications after waking to handle notifications being ready. Any app can use it to trigger a connection to their own server like Signal with empty content if they choose.
FCM exists to push data from the app's servers to their app on your device. Their servers must have the data to send it to it to you in the first place. The data being on their server makes it available to be obtained via lawful requests as is happening with FCM here. They can serve the warrants to the app's servers too, not only Google for FCM. It's easier for them to not need to deal with smaller companies but they can certainly do it for any large messaging app. This is something solved by E2EE, not avoiding a specific service because it complies with warrants requesting data.
Requesting data via warrants doesn't apply specifically to FCM. Google is requiring a warrant, so they're requiring more than Apple and it wouldn't be legal for them to outright refuse all the requests. If what you've taken away from the story is that you should avoid FCM, then you've largely missed the point. This applies to all data stored on servers under the jurisdiction of a country. It applies to all Apple and Google services, and all alternatives to those services. An alternate push messaging system can have the same requests made of them. If they use end-to-end encryption or don't support messages with content, then they can avoid content being obtained but they still have tons of metadata passing through. They could avoid recording metadata, but they are capable of recording it if a court requires that they do it.