If anything that happens in this book seem odd compared to the games and what not, these are versions of 21 that take place after Goku wishes her back, like he said he would at the end of the 21 story line (also the cannon story line of that game).

Side note, Jinzōningen Nijūichi-Gō is just Android No. 21 in Japanese lmao.

Back on topic, Good 21 was wished back, but since Evil would always be a part of her, she continued training for when her hunger would take over and Evil would return, she would be ready. When the time came, there was a lot of fighting, this time without the soul links. This does take place around the time Broly is fighting Goku and Vegeta so it was up to Good and Good alone. Being split from her other half they were equal in power, but Evil's hunger kept her distracted and with the power the Sayains were throwing around, Evil lost focus and was eventually losing the fight. When Good was about to finish her off, she stopped. Evil was confused as Good went to explain that Evil was scared, scared of dying again. Even if she would deny it, she knew she was right. Evil could only look down.
Good then held out a hand to the Evil her and tell her, "Its okay. We can overcome our hunger, together."
After thinking, Evil actually took her hand.
Over time they were able to conquer the Majin Saiyan appetite and now live normal, as normal as they can get.
Neutral 21, who came later on, was birth like Cell, being more or less a clone of the 21s before her. This is because 21 had set her main computer to create a clone of herself if she were to be defeated, which she was. So as Creator (me) checked to see if there was anything useful in her old lab, he met the lovable neutral 21 who was just hanging around as if she didn't know what to do and like she's been there for a while.
Now, The three 21s live together, with creator, and that's how we got here y'all.