That's true of current implementations, but the spec is flexible enough to allow much simpler ones. You can have an outbox which is a collection of activities (

That's true of current implementations, but the spec is flexible enough to allow much simpler ones. You can have an outbox which is a collection of activities (that could even be statically generated) and software could poll the outbox just like RSS. Existing software requires a push-based model, which means handling follows, which means a database, but fundamentally there's nothing stopping ActivityPub from being pull-based.