@plinubius my idea of a platform is very much from the POV of a software developer and entrepreneur.
I think for publishers, including individual amateur publishers, a *publishing platform* is a system you can use to publish and consume content -- text, video, audio, images.
I'm not sure if ActivityPub is a publishing platform. It might be more correct to say something more specific, like Ghost.org, is the platform. Or maybe more general, such as the Web, instead!