If I understand Smyth correctly, “Doric” is a bit of misnomer here and the term is used because the form is similar to some Doric forms, while this particular case is regular Attic. Or does this have something to do with the fact that Xenophon spent much time with the Lacedaimonians?
According to Smyth, “These are called Doric futures because Doric usually makes all futures (active and middle) in -σέω -σῶ, -σέομαι -σοῦμαι.” The LSJ article cites Lysias, Thucydides, Isocrates and Plato for future πλευσοῦμαι.