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.

It doesn't prevent trade just as effectively. Trips to Seoul don't pay for themselves. What do I care if my BTC might be worth more later? In the long run, we're all dead. I want to go on a vacation NOW.

What does a vendor care if your BTC might be worth less later? He's hungry NOW.