It would be silly for central banks to not be accumulating some Bitcoin just in case. They understand how money works..
It seems to me they are so conceited that they are not investing in bitcoin as you say, they are trying to regulate it and if they can they will destroy it so they do not want to be holding any of it, they are going to realize too late that bitcoin is not going to disappear and that is going to be one of the main currencies around the world, that is when they're going to try to get bitcoin but at that point people will prefer to use bitcoin for transactions instead of selling it for fiat.