Here "hearts" is a metonym for a person's mind. The phrase "hardened their hearts" is a metaphor for making someone become stubborn. Also, to "understand with their hearts" means to "truly understand." Alternate translation: "he has made them stubborn ... truly understand" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])