Purgatorio is definitely an epic poem. It’s in verse. It rhymes (in the Italian). It’s about really big epic topics like life and death, good and evil, God and the immortal soul.
Another huge clue is that the dude who takes over for Virgil, Statius, is—like Virgil—an epic poet. So, we have two celebrated poets guiding Dante, an aspiring epic poet, through the afterlife. It makes sense that Dante wrote Purgatorio in the same form as his heroes' works.