It is quite easy to underthink this case as well as to overthink it. A lot more than most cases, probably. But for my money, possibly all that has happened is that the German prosecutors received some quality intelligence from a snitch - or more than one snitch - about what happened to MM. Intelligence itself - e.g. "I was CB's mate, and he dunnit" - is not evidence usable in court. But it may lead you to evidence: you now know to look at where and how a named person lived, worked, travelled, operated.
So this intelligence about CB provided tangible leads for once: his name, odd bits of property usable as hiding places, his van, a local beauty spot by a lake.
This is consistent in particular with one quite important observation they've made - that HB is not core to the case. This would be so if what he had done was identify where usable evidence might be found. If so, the prosecutor's intended case is indeed not reliant on him. His value is that his information may lead the prosecutor to evidence, having which, they don't actually need him.
This probably wasn't information of the poor quality you get from some nutter who watches a TV documentary about e.g. the Suzy Lamplugh abduction. And suddenly "remembers" stuff about it with perfect recall fourteen years later having been completely silent in the meantime. No, this was better: specific places for police to go look.
I suspect that in the meantime, what's happened is that the promising new information has been looked into...and come up completely empty. There's been no word of anything found at CB's allotment shed, in his van, or at the lake. These are all places we know have been searched. But the tangible leads have all led, er, nowhere.
On a site like WS it is hugely tempting to imagine that murders can be fathomed out by the application of facts and logic. I suspect unfortunately that this is almost never true, and that what you need is either a smoking hot evidence trail, or, when it's this long ex post, a snitch. Perhaps all the BKA ever had was a really convincing and well-informed snitch, whose information has unfortunately not helped.
The BKA's communications strategy around this has been baffling. I really do not understand what they gained operationally from announcing, way back when, that they reckoned they had their man. It seems possible to me that what we are seeing now is them trying to walk back from their "CB dunnit" position and gradually dismount therefrom. If their source is flaking, they are quite right to say that this doesn't matter; he was never a witness, only an informant. The trouble now is that, assuming Germany has equivalent provisions to UK and other law around the right to a fair trial, it seems unlikely CB could ever be charged even if he did this, because he's been convicted by insinuation already.