We know from The Matrix Resurrections that unplugged humans can be reinserted into the Matrix, so Trinity was factually incorrect to say that Cypher couldn't go back.
The more relevant question then is why she thought he couldn't (or wouldn't) go back, and we likely have to speculate to some degree in that regard, since it wasn't explained in the film itself or any other official source, as far as I'm aware.
At that point in the film, we as the viewers knew that Cypher had made a deal with the machines to be reinserted into the Matrix in exchange for assisting in their capture of Morpheus, because we'd been shown the scene where that bargain was struck.
Trinity had no way of knowing about that deal, though, until he told her about it. Not only didn't she know about that specific deal, but as far as we're aware, she knew of no precedent for any such deal ever having been made between a human and the machines.
So, while it was obvious from Cypher's words and actions that he was betraying the crew of the Nebuchadnezzar to help the machines, she apparently hadn't considered the possibility that he was actually going to be rewarded by the machines for his efforts.
Rather, she likely thought he was behaving irrationally, and just lashing out against the crew out of sheer resentment and regret over having been unplugged.