That's a really good piece, by a really good philosopher who also writes extremely well - an anti-Butler, in so many ways. (One example: Nussbaum's 'The Fragility of Goodness'; Aristotle, Greek tragedy and how to live a life - and so much more. A wonderful book.)
If anything, she goes a little easy on Butler, I think. (Possibly strategically?) One example: "Nor is Butler’s treatment of Austin very plausible," says Nussbaum. Actually, Butler's treatment of Austin (on 'performatives') shows a lack of understanding that would shame a workshy first-year druggy undergrad. If Butler is as clever as people claim, this has to be wilful, and hence shameful, given her position and its obligations. More likely, we might think, especially in the light of myriad other expressed misapprehensions peppering her published work, is that she is not really all that smart after all.
Post-structuralism does offer bright clothing for certain ideas. Trouble is, the clothing in question is too often Imperial. Nussbaum quotes Butler:
"What does it mean for the agency of a subject to presuppose its own subordination? Is the act of presupposing the same as the act of reinstating, or is there a discontinuity between the power presupposed and the power reinstated? Consider that in the very act by which the subject reproduces the conditions of its own subordination, the subject exemplifies a temporally based vulnerability that belongs to those conditions, specifically, to the exigencies of their renewal."
"What does it mean ..."? We might well ask. We might well respond, "Fuck all". And we might well be right to do so.
TLDR Don't be taken in by Judith Butler.