I do not say that philosophy itself is irrelevant either to literature or the teaching of literature. I will say that there have been very competent philosophers whose judgment in literature is at least suspect--think of Kant quoting the poetry of Frederick the Great, generally regarded as pretty bad stuff--and that there have been very competent writers on literature whose grasp of philosophy isn't strong.
Shall I go home and remove from my shelves the criticism and poetry of T.S. Eliot, who never bothered to show up and defend his Ph.D. thesis at Harvard?
Picture a ven diagram where A overlaps with B, B overlaps with C, but AC does not overlap with A. Remove B and you have two circles that don't overlap at all.
Purge from your mind a whole aspect of who you are, and some other two parts become disconnected: uncontroversial to me.
Saying A relates to B and B relates to C says nothing about A relating to C.
Since the school chose an instructor with a philosphy background, they asked that he have an objective certificate of high achievement in that respective field. That was arguably better than asking for a Ph. D. in literature.