Federated systems tend to have different client and server implementations that can work in all combinations (as long as both are spec compliant).
The confusing part comes into play when the client implementation is a web app that you can use to log in to any server hosted anywhere. Because of how centralized services work, we are used to a paradigm where the website you visit hosts its own backend - you cannot choose. With these federated systems, on the other hand, you can go to website element.foo.com and log in at matrix.bar.com.
The blog post was saying they limited the option to log into your homeserver at matrix.bar.com (because of the reasons outlined above). This does not, however, stop you from hosting their (FOSS) app at call.bar.com and pointing it to your homeserver at matrix.bar.com
The confusing part comes into play when the client implementation is a web app that you can use to log in to any server hosted anywhere. Because of how centralized services work, we are used to a paradigm where the website you visit hosts its own backend - you cannot choose. With these federated systems, on the other hand, you can go to website element.foo.com and log in at matrix.bar.com.
The blog post was saying they limited the option to log into your homeserver at matrix.bar.com (because of the reasons outlined above). This does not, however, stop you from hosting their (FOSS) app at call.bar.com and pointing it to your homeserver at matrix.bar.com