At the risk of being "picky" myself(!), do you mean "knowing that they think" or "knowing what they think"? (I presume the latter)...
I don't think asking for clarification is picky at all, although I'm sure others could feel that way in a given circumstance. There is a clear distinction in my mind at least in what I "know" and "think". When I can quote peoples exact words I can restate what they say and post my statements on what they think or pose questions to their thoughts on the given issue, if I am wrong when I restate their idea they can clarify their own quote I am inspecting. I "know" there will be no confusion if both parties are interested in constructive discussion because we are interested in clarifying a specific issue. However I find that there are misunderstandings which can be very constructive and set up grounds for interesting discussion or thought but only once the other person clarifies their original stance.
Sometimes you get people reading a response then interpreting it in their own way without caring about what is exactly meant. Some actually go so far as to define your thoughts in terms of what they "think they know" (as opposed to "knowing you know" what the other person thinks which usually requires an exchange of elaborations on both sides until the situation becomes clarified somewhat) and then proceed to argue on that basis.
If they would find out what you think by asking for clarification there would be not so much argument and confusion. Some people respond to something that might be written in a general form and then make interpretations as to what it means and thus, even though it is nothing of what the initial poster actually believes, the responder tangents the conversation by restating what the poster thinks in terms of their own thoughts with no real consideration for searching for the truth, the initial poster then can start debating the wrongly interpreted idea and the process continues on and on. In the "theory of technique is dead" thread most posts do not actually define their generalized comments but each is responding to the other as if they know exactly what the interpretation of the generalisation is thus the thread go off into a spin of confusion with no real point to the discussion.
So I guess after all that typing I could sum it up by: What you know and think are two different things, arguments often happen when one mixes what they think with what they know. To know what the other person is talking about one sometimes needs to ask for clarification (there is no need for clarification if someone says something obviously wrong), through the clarification this sets us up for constructive debate. We must endeavor to clarify generalisations if they become an issue of debate, failure to do so merely causes useless argument.
....some people - barristers in particular, perhaps - can and sometimes do develop their skills in face to face argument to such an extent that they become quite alarmingly closer to written cut-and-thrust than most of us can manage; it's a kind of professional quick-wittedness born of the ability (essential for lawyers appearing in court) to see and present at least two sides of such an argument.
Yes I agree that there are some of us who can break up an argument and get to the main points immediately without wasting time during oral debate. The thing with face to face discussion is that you often argue a single main point and work around that. That single point may cause other questions to pop up but each one relates to the main question at hand in effective debate (ineffective of course tangents into irrelevance or less relevance). Online however and in writing, we tend to handle multiple issues at once, it tends to be more complicated but slower in the fact that multiple issues must wait for each response, like many downloads working at the same time
Often when multiple issues are debated many become ignored and focus is placed elsewhere. Irrelevant or small issue can become the main point of online debate because with the internet you have so many different types of people, nationalities (and I believe your native language effects how you think but that is an issue we could write a lot about and I will not suffer doing that here), devoting time to respond to all sorts of different things relevant or not.
There is no real judge jury or real observer physically in front of your to convince when arguing issues online, so being relevant really doesn't matter (of course one can get into semantics of what is relevant but that is irrelevant in my mind

If we had infinite time in our life it would be true that nothing is irrelevant, however we are mortals with limited time, best not to waste your time doing irrelevant things, unless you really need it.... I guess some people could need it who's to say they don't?! It's just ineffective, inefficient use of your mind and time but some people like that. All of us waste our time doing all sorts of useless things, the internet seems to have not escaped us observing people doing just that. I want serious discussion and want to constructively use my time, but I have to deal with those who just want to waste their time doing irrelevant things. But I don't hate these time wasting people, since I myself like to waste time just as all of us do, although when I procrastinate I try not to waste other peoples time!