he used it correctly, though most use it to describe something that won't dissolve into a liquid. It also means impossible to solve.
in·sol·u·ble
inˈsälyəb(ə)l/
adjective
adjective: insoluble
1.
impossible to solve.
"the problem is not insoluble"
synonyms: unsolvable, unanswerable, unresolvable; More
unfathomable, impenetrable, unexplainable, inscrutable, inexplicable
"some problems are insoluble"
2.
(of a substance) incapable of being dissolved.
"once dry, the paints become insoluble in water"
synonyms: indissoluble, incapable of dissolving
"these minerals are insoluble in water"