The future of kei/rw -- ke/rsw -- is unattested, but was made up by grammarians, as an "Aeolic" form, to explain the aorist e)/kersa; see e.g. Choeroboscus, Prolegomena et scholia in Theodosii Alexandrini canones isagogicos de flexione verborum p. 72. The actual future is kere/w (epic/Ionic), kerw= (Attic).
Though the form is given here in the third person singular, it has not necessarily been quoted from anywhere in that form; it might simply be used to match the Homeric aorist Choeroboscus himself was deriving (Iliad 10.456: a)pe/kerse).