I suspect Gjm's true argument is
Human interactions are complicated, there are usually multiple factors at play. It is true that from gjm's point of view I sometimes dismiss people's positions "arbitrarily". But it is also true that my style breaks the rules of the polite society in gjm's corner of the world and that makes him less comfortable. Plus there are status signals involved and the monkey brain is, of course, jerking around in response to them.
they may fail to be able to understand what your objection actually is
That's a fair point.
Gjm is also very uncomfortable guessing at people's motivations/reasons
Not guessing, but publicly stating. I am pretty sure that he -- like all people -- builds models of people in his head all the time. But bringing out these models into the open is too direct and explicit: gentlemen do not do that.
On the last point there, Lumifer is right and OrphanWilde wrong: I don't consider it in any way improper to build mental models of other people, and so far as I can tell I understand Lumifer's one-liners as well as anyone else does. (Which is not to say I always understand them correctly; but if not then his wounds are, as he might put it, self-inflicted.)
The other half of Lumifer's commentary, attempting to explain what I dislike about his posting style, is so far as I can tell quite badly wrong, but I don't think it would be productive to argue it further. (It very rarely is after one party has decided to go full Bulver on the other.)
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.