The Navajo language was genuinely outside their experience and their resources.

North American indigenous languages had essentially no presence in Japanese academic or intelligence circles — there was no historical relationship between Japan and the Navajo Nation that would have produced the kind of sustained language study needed to develop analytical capabilities.

The handful of non-Navajo people in the world who had real fluency in Diné Bizaad were almost exclusively Americans who had lived on or near the Navajo Nation, and they were not available to Japanese intelligence.

When Japanese signals personnel intercepted Navajo Code Talker transmissions, they reportedly found themselves listening to something genuinely unintelligible — not merely encrypted, but in a language whose phonological patterns and structural organization were unlike anything they had studied.

Some accounts, reported in postwar documentation, suggest that Japanese interceptors initially suspected the transmissions might be an artificial or machine-generated signal rather than a human language.

This was not a failure of Japanese intelligence capability.