The correct answer to this open question is the following.
What the newspaper reporter meant when he wrote that "flinty hearts were pierced and cold ones melted by his eloquence," was that Frederick Douglass gave a great speech full of meaning and eloquence. That is why the reporters used figurative language to express that the speech was so powerful that the stern people were moved and even passive people felt it. Douglass delivered his speech at the Massachusetts anti-Slavery Society Convention. Douglass had only 23 years when he delivered this speech on the island of Nantucket.