The German authorities may say they know how MM died. Sadly that may be the case. But without a body it’s impossible to know for certain unless there is overwhelming evidence to prove otherwise.
If CB didn’t do it, it’s possible MM is alive somewhere else totally unaware ( wishful thinking).
So far there is no proof that he's ever killed anyone.
Today's news from Deutsche Welle mentions the German police having digital proof that CB was involved in Madeleine's case.
Madeleine McCann: Police found files linking suspect with crime – report | DW | 18.06.2020
This doesn't mean that she is dead, it means they are linking him with the case. But if for example, they know he came back to Germany on say May 15 and without the child this is where they might assume the child is dead.