We can clarify how things work in a case like this.
A game needs to conclude prior to the cutoff time.
Rewards are based on a snapshot of the ladder that was taken at precisely 1:00.000 a.m. EST on Tuesday. Play results that were determined after that time are considered post-season results.
Several people above have correctly noted that the cutoff ratings for various levels of rewards usually go down when rewards go out, as players who are found to have misbehaved are removed from the final calculations. In this particular case, that may be enough to help the unnamed Death Knight over the finish line. But it may not.