If I understand it correctly, the claim is that unless something completely untrue - at least based on our current knowledge and experience, is stated, the discussion about the what-ifs of history must be distinguished from personal theories / speculations.
In my opinion, it is not that clear. For one, even in a good historian's opinion, there are inevitably elements of speculation here and there, whether he / she is a historian of the times he / she writes about or not. There is a multitude of things that are responsible for this, like the never absolutely complete knowledge of real facts and their context, some bias that may be present and as, in many cases, historical facts / events have been examined after they had happened, there is some inherent influence of seeing things in hindsight - even a little one. That is not to say that I don't trust good historians at all but that, in my opinion, we should take things with a grain of salt. After all, it is not always the historian's fault but rather that history is not described by some rigorous language and so, it can, on top of the above mentioned, produce ambiguous points and, equally importantly, no one can go back and see what exactly happened.
Now, expanding on this, a good historian what-ifs cannot also decline from the above. We can add to this that, as these what-ifs describe things that finally didn't happen, they can very likely give variables, along the way, values that are not at all sure that they would take not to mention the imponderable ones. So, after all, although a good historian can, no doubt, give some very educated and informed line(s) of thought, in my opinion, they still include speculations direct or implied, no matter of how low or high scale. While I don't discount them as approximations of correctness and accuracy of some degree, I don't think that they can give an ultimate "what would happen".
In the context of some such thread here at PF, I think that while it would be useful to discuss along the lines of pure historical events - given the constraints in my second paragraph at least in my opinion, the what-ifs, whether informed or far worse, not such, always, inevitably, lead to some sort of personal speculations and maybe, personal theories that after all, cannot be tested for their correctness given the irreversibility of time. So, although I wouldn't consider such a discussion the same as a personal theory in Physics or Math, which usually stems from misreading, oversimplification (see Pop Sci), misinterpretation or complete luck of knowledge of a topic / field of science, still, the personal speculation / theory element is present.
