In my opinion the secret is that the algorithm can be changed anytime. In fact there was a debate recently to amend bitcoin, which in the end resulted in bitcoin cash. So as long as the central bank controls the algorithm (i.e. can force by law or regulations miners to switch to a different algorithm), they have full control on the supply of money.

That defeats the original purpose of bitcoin, but that could be their intention. The settlement of the currency could be decentralised but the supply of the currency could be controlled.

In my opinion that leaves many other challenges, some fundamental like the concept of public ledger, incompatible with the secrecy of banking (i.e. the payments you make is not public information).