We've touched on it here before but another claim in GA's book relates to documents found in the Neuwegersleben raid.
He says they found 6 pen-drives and 2 memory cards which contained photos of children, including those of an unknown child in sensual poses (later discovered to be the daughter of CB's girlfriend at the time). Plus 2 Word document "books" which BKA describe as autobiographic writings authored by CB. The books are said to document serious sexual abuse against children and other criminal acts carried out by CB and friends of his.
If it's true, and CB really was dumb enough to document a number of crimes he was responsible for, then it's possible he has written of his involvement in MM. And it's possible that this account tallies with whatever information HB gave to SY regarding the confession.
It makes me wonder if one of the main tactics of this whole appeal was to find evidence and charge CB with a number of the other crimes first, which he may have also written about. If BKA can convict him for some of those other crimes, proving them using other evidence, it makes any defence argument that the book is just "fantasy" writing far less credible.
Could also explain a number of other comments BKA have made as well as their absolute confidence that CB is guilty.