Imagine you’re a Japanese cryptanalyst in 1944. No TV or wordwide network feeding you with knowledge et al.
In that case there’s only a minimal chance you’ve ever heard or seen a Navajo. So, you’ll be wondering what Joe is talking about as it sure doesn’t sound like English. You‘ll be asking yourself… Is that a failed decryption attempt? What’s the char frequency of Navajo anyway?
And since it’s a descriptive language: how would you know what is meant by (eg) “wolf feeds bisons when eagle moon embraces cloud rock”?
All in all it boils down to “security through obscurity”. In fact, you could als think of the Navajo communicators as “living codebooks” – which added an additional layer of security to the already used cryptographic protection during radio communication et al.