It is an accurate description of why it worked.

Understanding that accuracy matters because it points toward a genuine lesson about security: the most effective security often derives from obscurity and access asymmetry rather than from theoretical unbreakability.

The Navajo code was secure because the enemy had no path to the resources that would have been needed to crack it.

That is a legitimate and significant form of security.

It also means that the Navajo code was specifically effective against Japan in World War II.

A different enemy with different resources and different language capabilities might have presented a different challenge.