The answer is that although in terms of its emergence in the developmental chain Atzilut is not under Adam Kadmon, in terms of governmental control Atzilut is indeed under Adam Kadmon. A major principle on all the different levels is that the order of the developmental chain is one thing whereas the actual order of government follows a different arrangement. Thus we find that the Sefirot as manifested in the form of circles (עיגולים, igulim) emerge from and stand under one another. (For example, the circle of Gevurah emerges from and stands “under” – within – the circle of Chessed.)
Yet in the governmental order, Chessed, Gevurah and Tiferet stand in a row in spite of the fact that in the chain of development they stand one under the other. The same principle applies throughout the Kabbalistic teachings. In the case under discussion, Atzilut is said to stand under Adam Kadmon because it stands under and is subject to the governmental rule of Adam Kadmon through the latter’s Yesod and Malchut. (In terms of the chain of development Atzilut emerges from the branches of Adam Kadmon – i.e. from the radiance of Adam Kadmon’s face – but in terms of the governmental order, Atzilut stands under Yesod and Malchut of Adam Kadmon and is governed by them.)