Nah, Dieter was wrong. We as readers know that the story will be resolved eventually with everything coming to light (at least for us, maybe it won't all come to light in-universe). However, if this were real, it's quite possible that Anna / Nina remembering would be critical to revealing the truth and stopping Johan. As painful as it is, for the sake of bringing the truth to light, stopping Johan, and arguably giving herself closure, Anna / Nina remembering should not have been interfered with.