Is a two-opt move guaranteed to produce a non-worse tour?
2-Opt is a move that doesn't guarantee to give a better tour.

We use such moves ($k$-Opt, swap, insertion,..) in local searches to look for a better tour in the neighbor of the input (which is a tour).
Local search consists of browsing all (feasible) tours that can be produced with only one move from the input.
If the input tour is not improved, we call it a local optimum which may be the optimal solution.