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.

Yes, you're right. My statement was too strong. It still seems to me that its easier to ignore arguments online. In a real-time conversation you can remind someone that he hasn't responded to your argument, in which case he loses much of his plausible deniability. Online, such reminders seem to work very poorly, in my experience, to the extent that almost nobody even bothers to try them.

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'm not sure what exactly you are proposing here. Can you describe how you think the feature should work?

Does the link to epistemic hygiene (sorry for using a nonstandard term) resolve this misunderstanding?

Actually, no. Thanks for asking.

so it's a bad idea to fill your mind with hypotheses that you have no reason to believe

Isn't the fact that someone else believes in it strong enough to have stated it in public sufficient reason for me to put some weight into that hypotheses?

observing unexplained assertions is a pointless or sometimes even harmful activity

I can understand this if you mean random assertions, but I think that observing unexplained assertions made by others in good faith would be beneficial on average, even if sometimes harmful. Do you disagree?