On the NBC broadcast of the 10,000m, Salazar was quoted as saying that Rupp performed this workout this past week. Apparently, it's the fastest he's ever done this workout, and Salazar cited as a reason why the Flotrack Throwdown Mike wasn't a true indicator of Rupp's current speed (which is BS, IMO).
Also, what is the purpose of doing this workout within a week of a peak 10k? Wouldn't it make more sense to focus on something at race pace? Renato talks about how training is about funneling towards a specific goal speed, and 4x400 in 51 doesn't seem to accomplish that.