>In fact, in general, in any non one-on-one conversation the answer "I don't know" is not useful because if you don't know in a group, your silence indicates that.
This isn't true. There are many contexts where it is true but it doesn't actually generalize they way you say it does.
There are plenty of cases where experts in a non-one-on-one context will express a lack of knowledge. Sometimes this will be as part of making point about the broader epistemic state of the group, sometimes it will be simply to clarify the epistemic state of the speaker.
I personally will almost always say I don't know while talking thru to a solution. Admittedly this is informal speech that doesn't make it to written form.
This isn't true. There are many contexts where it is true but it doesn't actually generalize they way you say it does.
There are plenty of cases where experts in a non-one-on-one context will express a lack of knowledge. Sometimes this will be as part of making point about the broader epistemic state of the group, sometimes it will be simply to clarify the epistemic state of the speaker.