Exactly: own some. Not spend some, hence the problem with using it as a currency.

I'm not sure that is a real problem. You own some, then when you want to spend, you spend. The fact that it is deflationary means you might postpone your purchase, but ultimately, you will need to eat/travel/etc more than you need to "own" currency. Or are there other problems?

The issue is that, in a smoothly deflating currency, all but the most essential consumption will be postponed indefinitely, as will investment of that currency in anything that expects to return less than the rate of deflation. This is bad for an economy denominated entirely in that currency, as the deduction in economic activity can increase the rate of deflation, which reduces economic activity further, and so on.

Obviously bitcoin is not the primary currency for any economy, so the feedback effect isn't a concern, but the deflationary pressure is (per TFA) reducing actual economic activity denominated in bitcoin.

Pardon my french but what the fuck do you do with a number that you "own"? The whole point of a currency is to be spent as opposed to be "owned".

... why would you?

..because in this thought experiment it's guaranteed to be deflationary...

It's only deflationary in terms of bitcoins, the dollar value can still drop to zero if demand completely drops away.