This apparent impoliteness has a bit more to do with the
technical nature of the discussion. If I were discussing fine films or fine wines with someone I might let them get away with generalizations such as "the French certainly do make great ones". But as a programmer I could not let someone get away with the inaccurate statement "all operating systems use / as a directory separator".
If I were the quibbling type discussing films and wine I might be inclined to point out some of the lesser French films or wines so that my friend might not try to lead me into the falsehood that "all French films and wines are fine". But that would be irksome to my friend the Franco-eonophilic film buff.
To let the programmer in this example get away with their untruth without a remark would be somewhat impolite of me. It is not that I want to embarass or berate them personally that I feel compelled to point out the falsity of their assertion. I feel compelled to point out counterexamples, presumably because we are discussing code. Code differs from wine and films in that a computer may be asked to interpret it at some time. Hopefully at that time it will be correct.
Hopefully the human that wrote it has learned something about
the intracacies of computers by the time they "release" it. A strictly technical discussion is nothing personal really.