Again, it is polite, and encouraged, but assuming good intentions is for everyone…the observer just as much as the identifier.
While it is certainly preferable to include a comment, and also good etiquette (detailed in point 2, as you pointed out, and yes, the topic of this thread), there are many reasons why people might not do so that are neither “trolling” nor lazy. See here for some of them: https://forum.inaturalist.org/t/comments-made-when-suggesting-alternative-identifications-or-corrections/13823
Ultimately, iNat is a free platform. Nobody is being paid to participate, so people have their own legitimate reasons for how they allocate their limited time and energy while on it. You don’t have to agree or like their reasoning, but they are still valid, and you, as an observer, need to assume good intentions.
Notice, also: The people in that thread said they would reply when contacted to explain their disagreement.