It does mean they don't get upset if circumstances call for a change of plans. It also means they are willing to take experimental actions, ones that might not work out, knowing that in case of failure they can learn from the mistake and move on. Entrepreneurs are able to proceed on a project with only a half-baked plan complemented with an agility to change direction quickly to meet competition. Entrepreneurs have learned to tolerate ambiguity and their inevitable mistakes. Cultivating an entrepreneurial attitude helps you cope with change.
Many engineering schools teach the opposite.
The doctrine of requirements, specifications, prototypes, implementation, and testing implicitly calls for a complete, advance plan. Real engineering practice is not so well structured. The marketplace is too unpredictable and some requirements are unknowable. You have to learn to do planning in parallel with action, to write the specification as a byproduct of building your prototype. People who learn how to be comfortable with that will succeed.