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How could Rav ever be disproved?

Frequently, when the Gemara tries to disprove Rav from a Mishna, it answers that he's considered a Tanna and can argue on a Mishna.

But there's this case in Berachos, where Rav and Shmuel both say that one doesn't say a Mezonos on rice, and the Gemara concludes with a "תיובתא" against them.

Why doesn't the Gemara answer here that Rav is a Tanna?