The last paragraph of that segment says they pick it because it's the first in the list of idle cores. E.g. if a core is no longer idle it removes itself from the list, and it adds itself at the end, so effectively it's ordered in deminishing idletime time.
That said from what I grasp of the code it also doesn't bail out when power management is enabled, so maybe the public code or the paper are not at the same stage of development.
Though from my understanding of the patch they walk over all CPUs which is not precisely constant time, and it doesn't seem to be just the list of idle CPUs; for_each_online_cpu(_cpu): https://github.com/jplozi/wastedcores/blob/master/patches/ov...
That said from what I grasp of the code it also doesn't bail out when power management is enabled, so maybe the public code or the paper are not at the same stage of development.