Gotta be just a typo, Mike. As you say, blouwildebees are A LOT bigger than rooibok.

My bull wildebeest was about the size of a spike elk or a very big cow elk. My biggest impala isn't even close to the size of my biggest whitetail. Not even close. Impala are lightboned almost dainty critters.

Burger Cillié's "Mammals of Southern Africa" has males of the species as follows:

Impala; 103-180 lb
Black Wildebeest; +- 400 lb
Blue Wildebeest; 506-600 lb

I've never shot a blackwildebeest, but the numbers on the other two seem quite right for the ones I shot.