That said, this was a 3 judge panel decision (2-1) and they can be petitioned for rehearing en banc (meaning all the DC Circuit judges would sit). Not all of them are as willing to give the executive branch a happy ending as the two judges who ruled in the government's favor here. While it's not certain any of the parties in interest would petition, the DC Circuit itself can step in to rehear on its own.

In any event, the way I thought this would pan out was:

1. DC Circuit skeptical of district judge's posture but denies mandamus
2. Judge Sullivan hears out the parties, mainly to put DOJ on the record for having wasted his court's time (never underestimate how pissed district judges get when cases are inexplicably dismissed late in the process)
3. Sullivan dismisses case anyway, satisfied that everything is neatly tied up in a bow

We will see if the DC Circuit rehears the case (even odds -- they might do it because today's decision is a bit of an outlier, they may not because they don't want to touch this toxic bit of radioactive case). If the Full circuit doesn't rehear and change the decision, then what Judges Henderson and Rao did is create a big headache for the circuit in the future, as more and more cases seeking mandamus will show up. It's blindered conservative activism masquerading as deference to the executive branch.