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.