I'm seeing a lot of people expressing concern over the implications of this in relation to Akutagawa's arc, and while I do understand the concern... I also don't think it completely undoes or jeopardises Akutagawa's development, depending on the meaning behind Dazai's message.
I've already discussed this in a previous post, but one of BSD's recurring themes is this idea that your past does not define you, but you also cannot flee from it because it has fundamentally shaped who you are presently — and acknowledging those memories is the first step to healing from them. We saw this with Atsushi and the orphanage director in chapter 123: simply put (since I've already discussed this at length in that previous post), despite Atsushi having spent so long desperate to purge the director's voice from his mind, he's also never lived a life without his abuser haunting him — at the end of the day, the director influenced so many facets of Atsushi's life and has shaped so much of who he is today. (Hence why he looks horrified by the prospect of the director being gone.)
I'm bringing that up because it seems like Akutagawa's development entails something very similar, considering the parallels between him and Atsushi. Nothing could ever justify the pain and abuse Dazai inflicted upon Akutagawa, however the role Dazai has played in shaping who Akutagawa is presently is undeniable. Akutagawa's past does not define him, but he also can't flee from those memories because of how tremendous their influence on him is. He can't forget all he's endured to get to this moment, can't lose sight of what has shaped him — rather, his healing comes from reclaiming his past (a notion visually represented by him once again wearing the coat Dazai gave him during their first meeting).