Why would that matter? Apps using its api don't need to be AGPL
This doesn’t seem accurate. Isn’t AGPL viral across RPC boundaries requiring open sourcing not just the service but all supporting code for that service?
No it's not. From a practical standpoint, I'm not even sure how that could work. You would have to require all browsers to be open source AGPL in order to load a web page served by it. By way of analogy it seems the equivalent of requiring the mouse and keyboard firmware to be licensed the same as the operating system.
A real life example is Instructure, which makes Canvas (which is agpl) but has other proprietary services that interact heavily with it. It's never been a problem