I was under the impression that traditional Japanese soy sauce (shoyu, not tamari) also contains wheat (close to 50/50 ratio) - it's used to help start the fermentation.

It does, when it doesn't that's when you call it tamari.

Eh, it depends what you mean by traditional. Ramen is "traditional" in japan, but it was invented in the early 1900s. Similarly, since wheat wasn't commonly imported into japan prior to the 1800s, most actually old tradition recipes didn't contain wheat either.