Is the strictest manner of speaking no one can have certainty of the thoughts of others without reading their minds.

Socrates through Plato is as close as one can get, given his own lack of writing. That too suffers language and other barriers.

I don't personally see history of philosophy as primarily deriving value through perfectly accurate accounts of beliefs, but rather though exposing the range of rationals and justifications previously explored, for the benefit of ones own philosophical evaluations (or amusement).

Consider for example how early members of the academy had little consideration of the possibility of the lack of a free will, because the topics has little exposure and exploration at the time. Their writing might therefore sound less compelling to you than later philosophers.

Of course the stoics were determinists, but also compatibilists who emphasize the importance of choice.