Rabbi Shimon said Nedarim 58a): That Mishna is only referring to the halacha of bi’ur i.e. removing (the produce of Shemitah may be kept as long as that produce is still available in the fields for the animals; afterwards, it may no longer remain in the house).

The Ra”n explains: Since it was possible to eat it before that time, it is regarded as something that can become permitted.

The Sha’ar Hamelech and the Noda B’Yehudah ask: According to this logic, why is it only not nullified if it becomes intermingled with its type? The halacha should be that it should not become nullified even if it becomes mixed up in something that is not its type? For the Ra”n above (52a) explained at great length in the name of the Ri”f that something which is permitted presently to be consumed will not be nullified, even with a mixture of things that aren’t its type?

The Sha’ar Hamelech answers: The Ra”n only said that rule in respect of things that will be permitted forever, such as meat, which will always be permitted to be eaten with other meat or foods that are not from milk. However, here, the Shemitah produce will become forbidden forever, i.e. after the “time of removal.” It is therefore compared to something that will become permitted, where it will not become nullified only in a mixture of things that aren’t its type.