First, section 1031 is the explicit grant of detention authority. It no longer says anything about US citizenship, one way or the other. It is just like the AUMF in that respect. Of course, we need to recall that the Supreme Court in Hamdi had no trouble concluding that insofar as the AUMF provided detention authority for persons captured in combat in Afghanistan, that authority extended to US citizens (Hamdi left open the question whether the AUMF provided detention authority to other contexts, and if so whether citizenship would remain irrelevant in those other contexts). In any event, against this backdrop, section 1031 as currently written?and if examined in isolation?would not alter the somewhat uncertain status quo regarding the availability of detention for citizens. But 1031 does not stand in isolation. Consider section 1032.
Section 1032 is the supposedly-mandatory military detention provision?i.e., the idea that a subset of detainable persons (?covered persons? in the lingo of the statute) are not just detainable in theory, but affirmatively must be subject to military detention (though only until one of several disposition options, including civilian custody for criminal trial, is selected). Section 1032 then goes on, in subpart (b), to state expressly that US citizens are exempt from this ?mandatory detention? requirement (though lawful permanent residents are not).
This obviously rules out the idea of a mandatory military detention for US citizens. But note that it tends to rule in the idea that the baseline grant of detention authority in 1031 does in fact extend to citizens. Otherwise there would be no need for an exclusion for citizens in section 1032, since the 1032 category is a subset of the larger 1031 category.
So how does this compare to the status quo? Well, here we should probably distinguish between captures inside the US and captures abroad. Only the former, in my view, was still an open question (vis-a-vis the relevance of citizenship) under the AUMF.