A definite worldview means you believe the universe is deterministic. It does not mean that you believe that you’re omnipotent, which is the only way there could be no unknowns.
The difference between definite optimism and indefinite optimism lies in how you respond to the unknowns. A definite optimist creates long term plans despite the fact that those plans could be wrong (Definiteness means that you have a long-term plan.) An indefinite optimist responds to the existence of unknowns by not creating a long term plan at all.