Even if it’s an open gemara, that does not make it apikorsus either.
For example, if I were to say that R. Shimon ben Gamliel (instead of R. Yochanan ben Zakkai) told Vespasian that he was going to be the emperor, is that apikorsus? I would venture to say that it’s simply an incorrect statement and nothing more. Is 2+2=7 an apikorsus statement (since, I’m sure somewhere the gemara must add up two sets of two items each to come to a total of 4, such as by tefillin)?
In short, not every statement that disagrees with an open gemara is apikorsus. Wrong != Apikorsus.
The Wolf