What's important about future values is people's reaction to increased ROI on their investment. The pizza scenario is a perfect example. If you can buy a pizza today for say 0.1 BTC but in a matter of days (or whatever) that same 0.1 BTC could be worth $200. Arbitrary numbers but I'm just making the point that people's willingness to spend something that continues to prove that it not only will be more valuable tomorrow but is actually designed to be that way, very well could stagnant spending. As ones confidence grows that Bitcoin will be significantly more value next month than today, spending them makes less and less sense, from a investment stand point. Which is obviously the point the article was making.
It seems rather simple to me. You can buy pizza today for $20 and not spend BTC, or you can buy the pizza with BTC and use the $20 to immediately buy more BTC. The outcome is the same. Personally, I prefer to buy things with BTC and use my fiat income to buy more BTC because I know that my BTC purchases help the BTC economy which will cause the value of my BTC savings to rise, and because I think it is fun to use BTC. I am sure many others feel the same way.