Hopjokey......There are so many ways these shorts could have been covered. I'd like to believe that they simply were "smart" enough to cover in the midst of heavy volume. But, I don't.
Much more likely, IMO, they:
1) bought deep-in the-money calls from an institutional seller who had stock to deliver. There is no way to figure this out, of course. Add to this the fact that there are options transacted between large entities that never see the light of listing on exchanges.
2) Netted out a large long position that they also possessed.
3) Could have passed the short position to another entity on the last day of the reporting period. The new "owner" of the short position would have the 3 day settlement before it would become evident that the position did not decrease.
I have no idea how it happened but I do know that there are so many loopholes and ways around being caught in a short squeeze. It is sad that those who play by the rules have to hope that "they" make a mistake. As long as they don't err, they will win every time, IMO.