We know that chess can be solved, in theory. It absolutely isn't and probably will never be in practice. The necessary time and storage space doesn't exist.
Chess is absolutely not a solved game, outside of very limited situations like endgames. Just because a best move exists does not mean we (or even an engine) know what it is
This is a good example of being confidently misinformed.
The best move is always a result of calculation. And the calculation can always go deeper or run on a stronger engine.