Actually they said it as the current usecase of bitcoin but if we talk about future then it will surely be an alternate of Fiat Currency.
It was written that way but look what they did to their bitcoins, have they spent it? NO. Are they spending it? NO. bitcoin created a whole new class of an investment, though it was designed to be a medium of a transaction, people have taken the decentralization as a game for these coin's capability of rising and declining prices. We always say that bitcoin is an alternative currency, yet we don't use it as an alternative, maybe for some they do but most of us? hell no. We are afraid of the volatility, that's the truth.
I do not think that we are afraid of the volatility though I agree with you that most of Bitcoin holder does not use it as medium of exchange, I think people who are holding BTC have fear of missing out on profit if they use it as mode of payment. Meaning we are afraid of losing a possible gains once BTC price skyrocket. See this one of the reply as a proof.
As a payment tool? Bitcoin? Nah. I would rather use bitcoin as an asset for my investment instead of using it as a payment tool for transactions. I will use fiat that cryptocurrency because I don't want to waste my bitcoin. What's the point of you storing bitcoin if you will just spend it as an alternative for fiat currency? You should consider volatility before you make some decisions towards cryptocurrency. You can use ETH, DEX, and other altcoins for your transactions but not Bitcoin.