It's kind of along the lines of "premature optimization is the root of all evil." A true quote, that doesn't mean optimizing isn't important! The whole trick is in being able to tell what's core and what's premature.

Another way of looking at it -- scalability is pretty much the entire value proposition of software. It allows people to do the same, or more, amount of work without having to linearly scale out the number of people working on something. When you're first starting out, demonstrating even being able to _do_ the work is the first hurdle, but past that you're definitely going to need to show you can scale that out efficiently.