There is another gemara where Rav is refuted from a beraita, and the gemara does not answer that Rav is a tanna who can argue.
Take a look at Menachot 5a (today's page in the Daf Yomi cycle!). Rav is of the opinion that an asham metzora (a leper's guilt offering) which has been slaughtered shelo lishmo (with the intent that it not be an asham metzora but something else) is not offered up.
The gemara asks:
מיתיבי אשם מצורע שנשחט שלא לשמו או שלא ניתן מדמו ע"ג בהונות ה"ז עולה לגבי מזבח וטעון נסכים וצריך אשם אחר להכשירו תיובתא דרב
The Gemara raises an objection from a baraita: With regard to the guilt offering of a leper that was slaughtered not for its own sake, or if none of its blood was placed on the leper’s right thumb and big toe, this guilt offering is offered up upon the altar and it requires libations, in accordance with the halakha of the guilt offering of a leper. But the leper must nevertheless bring another guilt offering to render him fit to partake of offerings. This baraita is a conclusive refutation of the statement of Rav, who said that the guilt offering of a leper that was slaughtered not for its own sake is entirely disqualified because it did not render the leper fit.
(Translation and elucidation from Sefaria)
Tosafot ad loc. picks up on the fact that the gemara could have explained that Rav is arguing in his capacity as a tanna, but chooses not to.
הוה מצי למימר רב תנא הוא ופליג כדקאמר בכמה דוכתי
It could have answered that Rav is a tanna who argues, as it does in other places.
Tosafot does not explain why the gemara did not in fact do so.
Unfortunately, Alex's fantastic answer won't help us in this case, as it is only Rav's opinion here which is at stake, not Rav and Shmuel together as in Berachot 37a (the focus of the question).
An explanation is offered by Yad Malachi 150 (basing himself on Kesef Mishneh to Hilchot Ma'asei HaKorbanot 5:6):
אי הוה שמיע ליה לרב ההיא ברייתא לא הוה פליג אתנא
Had Rav heard of that beraita he would not have argued on the tanna.
Thus, the gemara will only invoke the principle of רב תנא הוא ופליג, that Rav may argue on a tanna, when it knows that Rav was aware of the tanna's statement, and chose to argue regardless.
However, if Rav did not know of the opinion of the tanna, as is presumed to be the case here, then we say that Rav would presumably have backed down were he to have become aware of it. Thus, the tanna's statment is an effective refutation of Rav's position.