This was true from, pretty much, Decade's second episode. My jaw dropped when Tsukasa established that he could just... speak Grongi for no reason. Oh, and be a better detective than all the characters who had been fighting Grongi for months.
Even at that, I think Tsukasa would come off worse had Rider not given us Tendou a few years before. I've honestly, seriously read KR fanfic SI Riders who were better characters and better-written than Tendou, whose bizarre infallibility quickly twists Kabuto plots into implausible, arbitrary messes.
(Tendou wasn't a bad idea, though-- the writers just needed to approach him and Kagami both with more subtlety and humanity.)