The footnote in Stephen Brown's edition, (Fran. Stud. 34 (1974) 200-295) refers us to Thomas commentary on book IV, IV, lect. 2 n. 558. In the old days you would have to look for some old book, probably in some university library. Now you can just follow the link. And it doesn't end there, because the commentary is linked right back to Aristotle's text.
[...] certain persons say that being is not the essence of a thing, but
nonetheless proceeds from the essential principles of a thing of which it is the
being, and is in the same genus by reduction with the thing of which it is the
being, just as motion is of the same genus by reduction with a finishing point.
Therefore the being of a substance is a single actuality in the genus of
substance by reduction, and is neither substance nor accident. And Thomas and
Giles hold this opinion.
What Thomas says is as follows. As I said, I don't claim to understand it fully.
|Sed in primo quidem non videtur dixisse recte. Esse enim rei quamvis sit aliud ab eius essentia, non tamen est intelligendum quod sit aliquod superadditum ad modum accidentis, sed quasi constituitur per principia essentiae. Et ideo hoc nomen ens quod imponitur ab ipso esse, significat idem cum nomine quod imponitur ab ipsa essentia.||558. But in regard to the first point he does not seem to be right; for even though a thing’s existence is other than its essence, it should not be understood to be something added to its essence after the manner of an accident, but something established, as it were, by the principles of the essence. Hence the term being, which is applied to a thing by reason of its very existence, designates the same thing as the term which is applied to it by reason of its essence.|