People's motivations and the extent of their knowledge, here, are not "somewhat speculative claims." They're conclusions that the historian reached by analyzing evidence.

What do you mean by "in the real world," anyway? How are people's motivations not part of the real world? "Islam" didn't triumph in North Africa; North Africans converted to Islam in overwhelming, near totalizing numbers (barring Jewish communities and with the exception of Egypt). That's fact. The history--the understanding of the past--lies in discovering why they converted. Which is to say, understanding their motivations and thoughts. And we do that by collecting evidence and analyzing it.

There is nothing off about this academic's work, any more than there is when an expert observer of current affairs says something like, "Theresa May clearly believes that some form of 'soft Brexit' is both desirable and possible'". These are common statements both about historical actors and present-day ones. As sunagainstgold points out, motivations are intensely "real world". A good deal of our neurological infrastructure is built for assessing the intentions and motivations of other human beings, both stated and implicit.

You might ask, how does a historian assess beliefs, thoughts, motivations and emotions, if they do not have the benefit of being able to directly observe a past individual's body language, facial expressions, etc., nor the ability to engage in some form of direct dialogue with a past individual? Largely we do it the same way that a contemporary observer of politics or culture does.

First, we look at what that individual said directly about their own thoughts, beliefs and emotions, in as many forms of documentation as we can find. With famous or powerful people, this research alone returns strong results. Not only do we find someone like Eisenhower frequently speaking to his own inner thoughts and motivations, we may find him saying some interestingly contradictory or variant things about his motivations at different times and to different audiences. Both the consistencies and differences in that testimony are relevant and useful. If Eisenhower says one thing about his motivations in a private letter to a friend and another thing when speaking in a press conference, a historian has an interesting interpretative challenge that gets easier to resolve the more material you have.

Second, we look at what other individuals say about what that individual said to them, and we look at what those individuals claim or say about the motivations of the person we are interested in. Someone like Eisenhower was surrounded by aides, assistants, colleagues, friends, family members, all of whom may leave behind evidence about what Eisenhower said about his motivations, and all of whom may have had impressions or perceptions of Eisenhower's motivations. I am not a telepath, so I do not know in a final way what my father's motivations were, but having lived with him for eighteen years and known him for another eighteen after that, I have a pretty good idea both based on my direct experience with him and on looking at all of what he left behind. I would be a good source if someone wanted to know my father's motivations. Famous or elite individuals have many such sources associated with them.

Third, we look at what someone actually did, at the actions they took. Motivations are often inferred, both in the past and the present. Specific actions tend to have a finite set of possible motivations. If we are reading the diary of a man who left a relatively poor household in Pennsylvania to go to California in 1849 and he does not speak directly to his motivations, we can still infer that at least one motivation was likely to have been to become wealthy or to escape his prior family or community situation. There may be clues that make that inference stronger in other actions that an individual took.

There's a deeper or harder problem that the OP's question might point to that historians do struggle with a great deal, which is how to talk about the mindset, emotions, motivations or consciousness of people in the past about whom we do not have a lot of archival material and who lived in societies that are very fundamentally different from our own. What was it like to be a person in a society that did not conceive of personhood in terms of the natural rights of legally and psychologically unique individuals, for example? But historians have devoted an enormous amount of attention to this problem over the last thirty years, and I think have succeeded somewhat in finding ways to address this issue.