I have read part of two of his works: The Pleasures of Philosophy (also known as The Mansions of Philosophy, I believe) and The Story of Philosophy. The work did not strike me as pure historical narrative. A lot of it was his thoughts on the place of Philosophy in intellectual history; the relationship of Philosophy to other disciplines; the nature of Philosophy; the nature of other disciplines in contrast to Philosophy; etc., etc. I would classify a lot of it as metaphilosophy. And a lot of his "history" of Philosophy seemed to be, rather than historical narrative, a part of his argument for his metaphilosophy.
Metaphilosophy is a branch of Philosophy, is it not?
I don't know if any of it was an "original treatise", but I am inclined to say that a lot of it was indeed philosophical speculation and not mere historical narrative. In fact, if my memory serves me correctly, the subtitle of The Pleasures of Philosophy was "An Attempt at a Consistent Philosophy of Life". That book, as I recall, was largely him articulating his own philosophy. Not exactly a history book.