The claim is that (1) if the value of a currency is unstable, it won't be useful as a medium of exchange, and that (2) whether it's unstable because it's rising or because it's falling does not affect the validity of (1).

The reason for (2) is that an exchange has two parties. If the value of your currency is cratering, it won't work as a medium of exchange because vendors won't accept it, which prevents trade from happening.

If, on the other hand, the value of your currency is rocketing, it still won't work as a medium of exchange because customers won't spend it. That prevents trade just as effectively.