In a real-time conversation or debate, this kind of plausible deniability doesn't exist as an option.

It does: you can often change the subject, give a non-answer or even be silent for a few moments and then try to continue the conversation without giving the answer.

• I request a reply.
• No reply is necessary.

Not just a reply, but a bare position statement (that's the right term, should also work as a signal for this mode: "My position [statement]: ..."), possibly without explanation.

I can't make much sense out of this sentence.

Does the link to epistemic hygiene (sorry for using a nonstandard term) resolve this misunderstanding? The point is that knowledge about assertions leaks through, biasing intuition about facts (blog:"do we believe everything we're told?", wiki:"Dangerous knowledge"), so it's a bad idea to fill your mind with hypotheses that you have no reason to believe -- it's knowably miscalibrated availability. As a result, observing unexplained assertions is a pointless or sometimes even harmful activity, but in this case it's exactly what is asked of the last-worder.