Ken talks about it in depth. Is there a part in there where he loses you? (by which I mean: a part where you're understanding it, and then in the next part you're not). I might be able to help you get your head around it.
Nobody is trying to get responders to shut up. They're trying to stop responders from shutting up the initiators. The distinction between the two is that the latter is usually qualitatively escalatory.
Alice makes a joke on Twitter, but rather than respond on Twitter Bob calls her employer. Alice schedules a talk at a university, but rather than scheduling a rebuttal Bob calls the university demanding to cancel the talk. Alice writes an op-ed, Bob publishes her address.
Both are speech, but a cultural norm of escalation makes the culture a pretty awful place to live.
FSP isn't an abstraction, it's a named bug.
Everything else you're saying is just Ken's point about proportionate response.
Second, I think you're missing what Ken called the "First Speaker Problem" (which IMHO is one of the most important parts of Ken's thinking)
> "We want to live in a culture where a joke on the internet doesn't lead to a struggle session at work"
The implication I'm hearing in those word is that the "first speaker" shouldn't be thrust into a struggle session at work for their speech; what I'm not hearing (although your literal words would cover this situation, so maybe I'm mishearing) is consideration into if the joke (or other statement) thrusts someone else into a struggle session at work.
IMHO, to re-use your words; it isn't batshit insane, it isn't gobsmackingly wrong, and it isn't that difficult to understand - whatever principles you want to apply to the responders to the apparent "First Speaker" should also be applied to the "First Speaker" as if they are themselves a responder. (If only because, I'd argue, they are meaningfully actually a responder themselves)