>Since human history is by definition what has been recorded
Isn't that just dabbling into semantics? There are numerous societies & cultures we can definitely prove existed, without having further info on specifics such as their monarchs, etc. Your statement on human history sounds like, "If a tree falls in a forest and no one is around to hear it, does it make a sound?"
It's still an instance in spacetime and our detailed knowledge of it doesn't delegitimize it.
Since human history is by definition what has been recorded, a small subset of what has happened, it is roughly possible. Granted, there exist records of monarchs whose deaths were not recorded, but we can still make pretty good estimates I think.