The meaning of a word comes from context, with a thesaurus you are meant to pick the correct word based on intended or derived meaning. The "synonyms" are not synonymous with all interpretations of a word. You can agree on a list of possible synonyms without fixing the definition.

The definition:

1. annoyingly insensitive or slow to understand.

"he wondered if the doctor was being deliberately obtuse"

> So I have to conclude..

hypothesis three: your argument wrt synonyms is incorrect. People may dislike being called "obtuse" but it is not, by standard, an insult.