These are still experimental flights. That could have been a bad procedure or a misconfiguration; we don't know. Claiming it as an upper limit if the design and then comparing it to a fully mature STS (you're quoting Endeavour's payload IIRC; Columbia was heavier and could not match its numbers) is cherry-picking. If you want to organise your claims with "in the absolute worst case..." then that's fine. But this is a subject with a lot of uncertainty around it, and asserting that one end of the error bars is the only one that counts isn't really justified.