Does this say you have more than 1 50 BMG rifle that regularly produce 1/4 MOA accuracy? What distance is this evaluated at?
Also, I believe the root of the OP's question was really weather one chambering could be more accurate than another. Certainly additional parameters are required to base this on such as range and as you correctly note the 50 has more "reach" which would likely make it more accurate at extreme range IMO
Mysticplayer noted that currently available bullets play a huge role in his opinion on the matter and I would concur along with the quality of brass available in 338 over 50 (not to mention factory loads if they are being used to evaluate).
I also saw mention of shooting a 20" plate at 3125 yards with a 338 (I assume Lapua??) The only way I could do that would be with some major horseshoe power and that would certainly not be on a consistent basis.