komponisto's tone would indeed be unjustified and not nice if his arguments had been rebutted and were only obvious to himself. As far as I can tell, nobody has actually rebutted komponisto's arguments, and a couple other people do think his view is obviously right.

orthonormal gave the following objection:

No, what the teacher means is "I will act as if you had not turned in anything at all", and "I won't grade it" is a perfectly reasonable shorthand for that.

The argument is that the equivalence in meaning between the two phrasings is so close the two are interchangeable. komponisto rebuts this argument:

(a) The first time I heard it, I didn't realize what the teacher meant. ... (d) Because people should say what they mean and mean what they say.

Later, SilasBarta argues that non-neurotypical people might be confused by such oblique language.

Right now, komponisto's position lacks a convincing rebuttal. There could still be counter-arguments (e.g. "people who find 'will not be graded' confusing are atypical and they don't matter", "people who find 'will not be graded' confusing should learn to make inferences and detect euphemisms, because these are valuable skills"). But nobody has made any such potential rebuttals, unless I'm missing something.

(And there are rebuttals to those rebuttals: "people who have trouble with euphemism-detection deserve accommodation, not marginalization", "euphemism detection is a valuable skill, but in the student-teacher relationship, clarity of communication is more important than teaching that skill.")

You offered a potential rebuttal, but it hardly closes the case:

Obliqueness is an epidemic, but you seem to be drawing the line very uncommonly. I would be only a little more surprised if you had chosen to rant about someone expressing an intention to turn on their lawn sprinkler, saying that this is objectionably oblique because what really matters is that the grass will get wet, not that it be delivered by a particular device.

I don't think it's uncommon to hold certain types of communication to a higher standard of clarity, and communication of expectations between a teacher and student may be a good example. It's a good thing for students to feel that the teacher is talking straight to them about potential punishments that could effect their futures, rather than talking in euphemisms.

It actually is perfectly fair for komponisto to query why people aren't conceding the point, since his argument lacked real rebuttals at the time (and still does). To me, it sounds like he is saying "agree with me or show me how I'm wrong." Since he seems to actually have grounds for being confident in his position, his tone reads as passionate to me, rather than an "not nice." If someone shows some stronger arguments against his position, and he persists with the same tone, then I would say that he is being "not nice."

Several people have been rubbed the wrong way by komponisto's communication style in this discussion, which could well be evidence that it could use improvement. Perhaps if komponisto had spent more time eliciting counter-arguments before arguing that people should agree with him, then he would have avoided that interpretation. Yet calling him "not nice" seems to require acting as if his argument has holes in it which haven't yet been shown, and requires a particular interpretation of the socioemotional content of his posts that I don't share:

Also, you're implying that people are actively defending the usage of language to which you object, which seems to me a mischaracterization.

By saying that "'I won't grade it' is a perfectly reasonable shorthand," it does sound like orthonormal was defending the acceptability of the oblique phrasing. If komponisto is mischaracterizing orthonormal, let orthonormal be the one to say so.

Now you are insinuating that disagreement with you constitutes flouting those values, which is insulting and kind of a cheap shot.

The question we should ask is whether is it reasonable for komponisto to believe that his position on this subject is so strong that he is justified in considering disagreement with him to be flouting rationalist values. As far as I can tell, his position might be strong enough to justify such a level of confidence. Yet I am not convinced that it is, so I think his meta-comments about rationality on Less Wrong would have been better saved for later. I'm willing to call his comment overconfident, but I'm not willing to call it insulting or a cheap shot.

This is just an attack on orthonormal, whose comment was not particularly objectionable in any way except inasmuch as it attempted to correct you.

I disagree that "Exactly what mistake do you think I'm making, all ye hordes-of-orthonormal-upvoters?" needs to be interpreted as an attack. While I wouldn't object to orthonormal's comment, his phrasing wasn't the gentlest.

You are not obviously right to people other than yourself. That doesn't mean you're wrong; but it means you can't get away with this sentence and sound nice.

Yet after you made this post, some people have appeared (e.g. Silas) who do think that komponisto is right (though they think the point is trivial), so perhaps he can get away with saying something like that without being not nice.

Personally, I would have preferred to see a bit more back-and-forth on the substantive issue in this discussion before either of you attempted to go meta (komponisto about why people were disagreeing and their rationality, and you about him being "not nice," or being the only one who holds such a position).