Since Patrik Korda still criticises my approach to money substitutes, I thought I'd summarise here their fundamentals. This is not a definition of a money substitute, nor an exhaustive listing of all possible types of substitutes. Rather, it is to explain their essence in a way that is easily understandable and clarify why Bitcoin has nothing to do with them.
The fundamental of a money substitute is a causal link between money in the narrower sense and the substitute. This causal link is the main, necessary, factor explaining the price of the substitute. For argument's sake, I'll leave it open whether other, optional, factors exist. This is the essence of the point Mises was making, he was trying to explain the price of money substitutes. And he found the explanation in the causal link to the money in the narrower sense.
For Bitcoin, there is no such underlying causal link to a money in the narrower sense that explains the price of Bitcoin. The price of Bitcoin is determined by other factors.
Therefore Bitcoin is not a money substitute (or it wouldn't be if Bitcoin was money).