A lot of startups jobs are well-defined and have a pre-existing business strategy. In fact, in my experience, startups can even be much less fluid in this sense than big companies, because startups are funded for a much narrower mission based on a very specific vision, whereas big company often has a huge scope within which there's tons of ambiguity as to what to pursue.

A lot of these types of statements generally are made by founders whose relative role at the startup (C-level) is much senior than their past or would-be role at larger companies (entry-level or close). Or early employees who got to be much more senior within a startup than they had been at a larger company. This just isn't an accurate statement for the rank & file or when you compare people at similar levels across companies.