Something worthless can be used as money in the same way as a piece of unjust legislation can be used as law. Its use does not prove that it is actually a real measure, only that irrational men have decided to use it.
The following is speculation that may be absolutely wrong:
However, there is a sense in which they do act as measures, but only accidentally. A bitcoin is used as money because it is measured by other things, which gives it an accidental character of a measure, though in itself it is no measure. Much like an unjust law is only accidentally used as a measure of justice, because some measure of justice is annexed to it accidentally from elsewhere.
Legislation protecting murder is a measure of justice only accidentally. It has justice annexed to it in some way or other, but is essentially unjust and thus no law at all. This is the case of abortion protection, where just adheres to it only accidentally from just laws protecting “privacy” or some other real law. It can thus take on the character of a law and justice, while in itself remaining unjust. It is in this way measured by other laws as having the an accidental character of justice. This is part of what makes it appear to be a real law.
Bitcoins similarly have this accidental character of value. They are measured by other things which have real value. Through this measurement, they take on the accidental character of value. Bitcoins are valuable purely because they are exchangeable for other things, not because bitcoins are valuable in themselves, but because a sort of value adheres them through extrinsic measurement. They are no money in themselves, but takes on the accidental character of money.
A particle at 0 Kelvin is no measure of motion. However, as measured by other motions takes on the character of time. In itself it cannot measure motion, but becomes a measure by first being measured, and thus only accidentally.