In no way is the person you're responding to speaking defensively. They've discussed the reason why your extrapolation to a full-mesh connective worst-case scenario isn't based in the reality of how ActivityPub functions. But you don't seem to be willing to entertain the notion that the federation of any given action never exceeds the number of instances subscribed to the community that generated it.

Even should every instance subscribe to every community on every other instance, the recipient of a federated action doesn't turn around and rebroadcast that action back on to the network because it is not the authoritative host of that community. Therefore what this discussion is lacking is proof of where this exponential broadcast storm of federated actions comes from in your assertion.