Since R' Yochanan is explaining the view of the Tanna R' Yehudah, he has to explain the case as not involving bereirah since R' Yehudah does not hold by bereirah. How do we see from this that R' Yochanan himself does not hold by bereirah? - it is possible that he does and just explaining R' Yehudah
Avraham Sacks, Ramat Beit Shemesh
Sholom Rav.
The Gemara generally assumes that if an Amora (or a Tana) explains the statement of a(nother) Tana, it means that he holds like him - unless elsewhere he specifically holds otherwise - in which case the Gemara will answer that he is merely explaining his words but he does not really hold like him.
See for example, the end of the Masechta - the bottom of Daf 30a.
Be'Virchas Kol Tuv.
Eliezer Chrysler