Firebase at it's core is just Google Cloud, same with AWS Amplify just being various AWS services neatly delivered. I imagine some people are using the exact same services just through Google Cloud and AWS. Cloud Messaging == SNS, Lambda == Cloud Functions, Firestore == DynamoDB etc.

I think you might be underestimating their reach though, they're very popular in app development shops, since an app developer doesn't need to know how to build a serverside application in order to get an app that needs remote services working.