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.
This is good advice. I've scaled multiple sites to +3 million registered users and more than 1 million unique visitors with heavy DB usage. This is excellent advice for starting out. Along the way we found areas that could be improved and we were able to move quickly to them because our underlying DB was standard stuff. We knew what areas were breaking down and then fixed them with targeted moves to other DBs and things like memcache. Yes, each DB has a specialty but a good general DB gets you a lot of mileage because those specialty DBs do very little a general DB can't but that is often not true the other way.
You would never start a journey with cloudy destination and of a million miles by asking for a moon rover as your vehicle. As your journey develops you would start looking at other options or options for certain segments. Apps follow similar patterns.
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.
