matchboxbananasynergy I think that "attack channel" is very strong and scary wording. I'm assuming that what @evalda means here is that if an app is not properly using FCM for notifications with Play Services (Signal is an example of an app that does this properly), you could be leaking your notifications to Play Services.
I am sorry if it sounded scarier than it should be :) Scenario you describe is one possibility, but I was referring to the fact that any inbound communication channel (push notifications in this case) create a new way how your device can be targeted and potentially compromised.
I don't think it adds a lot of risk, especially for an average user. Nevertheless, it's still an extra communication channel which may have bugs in its implementation which can be discovered and potentially exploited. The less ways there are to receive network packets from the outside world, the better. But again, I don't think it adds significant risk.