All created things belong to the "natural order" because, as it has been rightly stated, only God is SUPERnatural, that is, "above (created) nature".
Angels may have more and better natural abilities than we do, but even as animals and men, or even plants and men, being as far apart in gifts and abilities as they are, belong to the scope of (created) nature, so, too do the angels belong to that same scope.
Yes, St. Thomas is, by the way, at least one of the people who said that only God is supernatural, or "above (created) nature".
That the angels are far superior becomes useless as an argument when we realize that even inanimate objects are put in the same category with human beings, under the "natural" heading. And there's certainly a wide gulf between us and the stones. Just like there's no "sub-natural" level to define created things that are way below man, it also makes sense that the angels would be included in the same scope of all things created.
It seems St. Thomas uses the words natural and supernatural here, to define that which is created, and subject to some constraints of it's nature, and that which is uncreated, and subject to no constraints (of what could be called God's nature). [My personal understanding.]
Of course, if, by nature, you mean that which is intrinsic to something, then even God has His nature, so to speak. But speaking of "natural" as that which belongs to the created order, then of course angels would also be included in that category, in spite of their awesome abilities and gifts from God, because they, too, are created, even if their nature has such superior properties and gifts to our own.
[Sorry if my post is rambling. I've got a bad cold, and it doesn't do the brain any favors.]