I thought this was sweet!! And while I know it's not quite an accurate representation of DID, I understand that the author wanted an ending that didn't overwrite Tae-Oh's existence even if it's not entirely realistic, and there's validity in that because it'a a fictional story with the intent to entertain not to educate. I'm sure there was also an element of not wanting the story to end too similarly to how Double Trap ended, too, since their stories are already very similar as it is. Dongjae choosing both feels like a fitting development for his character and it made sense to me, especially since all three of them are well aware that this relationship will have its fair share of problems but they love each other enough to try anyway.
The top really took a crash course on gaslighting from his twink uncle and now he's a master of the art omggg