Possession is just one of the many meanings of , and does not apply in this case. here means **** , i.e. "Hino is an idiot" (as you suspected).
See: How does the work in ?
While this substitution mostly applies to relative clauses or double-subject constructions (unless there is a direct object marked with ), the subject particle can be replaced with in many cases.
Refer also to the Jisho definition for
_1\. indicates possessive (...)_
_2\. nominalizes verbs and adjectives (...)_
_**3\. substitutes for "ga"** in subordinate phrases. (See also ): There were almost 100 people, few of whom I had seen before. (...)_