That is a distinction without a difference, mostly - since those paying customers are free to then publish the source code they got from you for free for everyone.
The theoretical possibility of charging money for distributing GPL software has not materialized for any company. You can get RHEL for free if you want, for example, and build sell your own devices with it.
You can't charge money for distributing GPL software because with high speed internet that is very cheap and easy to do. Competition will and has brought that cost to about zero. And that is a good thing. Everyone now has free access to a huge amount software with source code.
In software support, training, and adding/fixing bugs for a fee. Redhat was a large company that did this.
If the world stabilized on a few software products that did what people wanted to do, like and office suite, that would be great. No more work needs to be done on it. Just a little maintenance going forward. Like roads. They cost a lot to build, but little to allow someone to drive on them.
The theoretical possibility of charging money for distributing GPL software has not materialized for any company. You can get RHEL for free if you want, for example, and build sell your own devices with it.