I feel like this a false truth.
Most people are building a web app, or a mobile app and consuming an api and displaying all the data they retrieve.
If you have an rest api which returns an object with 6 properties. And a graphQl scheme which returns those same 6 properties. You’re not saving anything.
Now if you have a website and a mobile app, where the mobile app needs 3 fields and the website needs 6 fields. You will obviously save on bandwidth with the mobile app.
The problem here is most of us are not building Facebook. The data saved is peanuts and the bandwidth cost is probably going to be far less than the total cost of doing the work to support graphql.
For a company like Facebook which has many different integrations as well as 3rd parties integrating, graphql is obviously a godsend as integrators and integrations can consume only what they require and save Facebook millions of dollars in bandwidth.