It looks not only legit, but rather unimpressive as well. I mean, i've seen WB hitting for over 10k, some 15k+ heavy attacks, and I know even those are legit.
if he hit all those SA in 0.5s I would call it impressive
A death recap doesn't really tell us anything, a screen cap of a combat log with time stamps would be more useful there. If he hit you 5 surprise attacks over 6 seconds, its totally legit. But if all 5 of those hit you in say 2 or 3 seconds then the GCD is being accelerated.