By abstract here is meant ideal, in the Platonic sense: thus ens abstractum answers to auto to on. It is not abstract in the sense of indeterminate: it is not that thinnest of abstractions, being in general. It is being, sheer, simple, and full. See Chap. XXV note a, XXVI. In fact ens abstractum here is tantamount to ens perfectum: cf. the argument about 'perfectum bonum, III, Chap. XLVIII, 5.
But probably this argument is not St Thomas's at all. It is wanting in the Bergamo autograph in the Vatican library.
Of God and His Creatures: 1.42