Yevamos 33a - Is R' Chiya a Liar?

The gemara uses a very strange phrase. It seems that the assumption of the gemara is that when there is a machlokes that stems from a mesorah i.e. whether Rebbi said you are chayev 1 or 2, the rule is that both positions have to be justified. Meaning each opinion has to explain not only why he is correct but also pinpoint the mistake of his colleague bec. otherwise he would be claiming that his colleague is a liar by saying over a wrong mesorah (to the exclusion of an argument in sevara where each side presents its logic). Someone in my shiur asked if this is a standard rule that anytime 2 opinions argue about what they heard from their rebbi, does each one have to justify how his colleague could have made a mistake? We find many places where there is a machlokes about what someone said (like between r' avin and r' dimi) and the gemara never makes a big deal of how each one can claim the other is a liar?

Someone else suggested that it could be that only here where they supported their mesorah by taking an oath, does each have to justify the opinion of his friend.