It makes sense that Aqua would hide his past identity after Ai died and he started down his dark path of revenge, but prior to that it makes no sense that neither of them ever discussed anything about their past lives. They literally spent years only able to have real conversations with each other but, apparently, they somehow managed avoided ever talking about hospitals, cancer, doctors, etc.
I initially saw that as this story's biggest flaw but accepted it as a sort of artistic flair that would never be acknowledged like a reverse Chekhov's gun, so I have some really mixed feelings about this chapter. It either should have happened way way earlier or not at all/not until the end (wait... what if the story is ending. That'd be funny)