What do you mean by traditional humanities? If you mean literature and skills like scripting out plots for characters and artful language in prose/poetry, then sure, it's not essential to business writing. However, if you're talking about essay writing, persuasive writing, clear language, laying out complex arguments, etc. then it's very, very relevant to business writing. Both categories are part of traditional humanities.

Getting to the point on nuanced issues like strategy or design is actually quite difficult because you have to both understand your point well enough to distill it and you have to be good at putting the words down. I'd argue that the "understanding" part is harder because, from my personal experience, someone who is thinking clearly and just not fluent in English still organizes the writing clearly, but someone who isn't thinking clearly will produce great syntax but the reasoning is hard to follow.

I see much more of the latter than the former in my day to day.