That's a totally valid use case.
Bitcoin can be many things simultaneously.
If by strictly, he means solely, then something that is strictly an investment vehicle makes no sense. The piper must be paid at some point.
I can invest in shares of a company, but the shares aren't strictly an investment vehicle, they serve as a way for the company to acquire capital to put toward it's mission.
It doesn't require a 'mission' to have value- the fact enough of society (still a small minority which actually invests or knows the price) recognizes that others recognize it has value allows it to fulfill exactly this function
If we weren't using gold in this way the price would indisputably be a fraction of what it is vs if the industrial applications stopped it would definitely still retail value.
Bitcoin is limited to 21M coins and we are at 19M of them. It is functionally more scarce than gold in that once we hit 21M that's it- we can't develop some new technology that unlocks more gold like asteroid mining.