I don't think they're going to start adding firestick numbers to Xbox.
If they did that, then they'd also need to add all other devices that run Xbox Cloud Gaming and then they'd have billions of Android, iOS, ipadOS, macOS, Windows, Xbox One, Samsung Smart TV and ChromeOS devices also counting as "Xbox Series".
This FireTV compatibility is just one more device that can run a Cloud Gaming client, among many that were already available. This news is a lot less exciting than some are making it to be.