Well it says 40000 kcd, which does convert to 400m of throw. It’s also backed up by zeroair’s review: Convoy M21B Flashlight Review - ZeroAir Reviews. People in this thread estimate the M21B with Osram is actually capable of 100kcd+.
Your title says S21B while your link is for the M21B. I couldn’t find measurements of the S21B’s performance with SST-40, but I believe the S21A uses the same driver and reflector and was measured at 319m in Flashaholic’s review. The S21B with Osram was measured at 480m by 1lumen.