I think it makes a big difference that the topic "acts" like a regular "user" account but is stewarded by potentially many users. Why don't you just name this "channels"?

Probably because, differently from channels, our topics include the possibility to be grouped in a taxonomy tree, so that eg: the "All about veganism" topic can be a children of "nutrition" and parent of "tofu recipes" topics (Nd such taxonomy is also federated, so the tree can span different instances as well once we get it)