I did look it up, even before posting the first time.
I see that Google does list the definition you suggest. But it does so without attribution, as far as I can see. Dictionary.com (the place I had been looking), which takes definitions from several dictionaries, with attribution, does not offer the meaning in question.
You said "many dictionaries" would list this meaning. I think your claim remains unsubstantiated.
Clarification: the Google page for "obtuse" does list a source for the meaning "hard to understand", but not only is that source not a dictionary, it offers the meaning only to say that using the word with that meaning "is not considered good style".
