In this case, frigate means a ship because the poem summarizes that a ship cannot take you as far as a book can in literature. Coursers is referring to a horse, and there’s a context clue to support that; the word “chariot.” A chariot is a two-wheeled horse-drawn vehicle that was used a lot in myths and fables. The poem says that there is no ship (or form of transportation) in literature like a book, and that there is no horse that can take you away like poetry. If coursers did not mean horses, then the comparison between a page and a courser would not be personified.