The Gemarah asked why are R. Meir and R. Yehuda arguing over the same thing twice (Gezeiras Tikrah) and R. Chiya bar Abba amar R. Yochanon suggested that the 1st Mishnah is arguing about Gezeiras Tikrah. The Gemara rejects this based on something R. Yehuda amar Rav said and than gives a different answer. All of these people are Amoroyim, is Rav so much greater that he can upshlug R. Yochanon?
If you say that the Gemara doesn't reject R. Chiya bar Abba amar R. Yochanon answer, but is asking how would R. Yehuda amar Rav answer this, that would be good. My question was based on Artscroll explaining that the 1st answer was rejected.
There are two basic premises about Rav Yehuda Amar Rav's statement which the commentaries explain to answer your question. One is that it is clear that Rebbi Yochanan can, and should, agree with this statement for various reasons (see Ritva and Pnei Yehoshua here at length). Additionally, the Pnei Yehoshua adds that this statement of Rav was "Masim b'Chol Yom" meaning that it was accepted Halachah, and therefore sound reason to seek a different explanation. I am quite sure that the Gemara will sometimes quote an accepted Halachah as a valid question, even if the person it is asking on could technically argue on its author. Being that we would like his statement to be as (Halachically) valid as possible, it is enough of a reason to search out a different explanation.