I agree that questions about the history of Unix should be on-topic.
I agree that the help/on-topic page is vague on the subject,
and it wouldn't hurt to clarify it. Or not.
I'm not sure we need to change the help page,
since it doesn't say anything excluding such topics.
I think you're mixing apples and oranges.
When did directories stop being readable as files? is a great example of a history question that's appropriate for our site.
Why isn't Linux embraced as the official GNU kernel?,
on the other hand, is a question about why people chose to do (or not do)
certain things. Sure, if Linus Torvalds has an account on Stack Exchange,
I'd be interested in seeing what he has to say.
And I suppose that a link to a statement that he's made would be valuable.
But questions like this seem to be most likely to attract guesses
and opinions; therefore, I consider it to be off-topic.