This is absolutely correct. Proving P=NP doesn't magically create the algorithm, it just proves that there is one.

However, the reason why I chose to single out crypto specifically is because it has the most to lose if that algorithm exists. Our current methods of encryption become unsafe regardless of whether the algorithm is known or not. I don't think you can claim that your encryption is secure if there is an algorithm that can crack it in polynomial time, regardless of whether the algorithm is known or not.