The point is that we know that at one point Ikuko and Tohya started writing forgeries about the Rokkenjima incident.
We know that the forgeries describe details that only a person who was actually there could know.
So it follows that Tohya provided all those inputs and that can only work if he remembered them.
Why would Tohya decide to do that if he really wanted to forget everything? It's clear to me that despite his fear of becoming someone else he was still obsessed enough to want to know more about those memories he has in his mind.
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