Earlier quoted context omitted.
I don't think this is terribly good advice. Many databases have specific domains in which they excel. I agree you shouldn't be worried about scaling before you have users, but you should absolutely evaluate different tools before picking one.
What you're saying is true, but worrying about scaling before it's necessary is unfortunately common, and I've felt the pain caused by this (modern) premature optimization.
This tends to be true of all things.