I generally read "optimal" to mean "optimal according to some objective function". In the case of a technical design, the objective is not something you can readily formalize, because there are so many considerations.

For example, suppose you are designing a network service to meet some use case. Yes, it needs to meet the use case, but there are other things you care about like simplicity of the code, latency, cost to operate the service, and others.

For any design you propose, there might exist an alternate design that is strictly better by some criteria-maybe it works exactly the same but has much better latency characteristics. If an oracle gave you this design, you might agree it was more optimal and choose to implement it, but I don't think it means you failed if you come up with a (sub-optimal) design that solves the problem within the constraint.