Complex sentence: The poem is important for the roll call of poets it contains, some of whom we know of only from their citation in this work, so that it stands in part as a poetic testimony to historic loss in literature.
Split into simpler sentences: The poem is important for the roll call of poets it contains, some of whom we know of only from their citation in this work. It thus stands, in part, as a poetic testimony to historic loss in literature.