As another commenter mentioned, it's at least a good marketing campaign, causing me to open the app for the first time in months. Or it might be to gather a baseline of how many people react to their notifications, regardless of their contents? (Perhaps to immediately go to the notifications settings to disable them)

Edit: if the notification is sent by an attacker, does the attacker get access to firebase metrics as well? (Guessing they at least contain info about how many people opened the app)