That's the rule. I disagree with the rule but the rule as called in the SB is exactly how it was called against Pitt in Indy (by Leavy's crew BTW) and the next week against Pitt again when Cinci intercepted the ball. I hate the rule, I think it's poorly written because it ignores the idea that a tackler might need to go through a blocker to make a tackle, but that's the rule and it was called according to the rule.
See "Tuck Rule" Patriots-Raiders circa 2002