Why does TJ skip an entire millennium of philosophy and history? Even if he wasn't particularly clerical, there's a whole bundle of republican history from Holland, Novgorod, Switzerland, and northern Italy to be learned, plus everything from Ockham and Bacon to Aquinas and Boethius.
I have no answer for your question. Perhaps somebody on this forum may present their knowledge on the matter.
Northern Italy - Are you referring to the history that surrounds Machiavelli’s writings? Some of his cited political acts amazed me.
I would also point out that for the 18th century, ‘modern’ history did not have the narrow ‘post-1648’ meaning it does now (and has had since at least the late 1920’s or early 1930’s). Their understanding of ‘modern history’ would have encompassed what we now call ‘early modern Europe’ including (usually) the Renaissance and Reformation, rise of the Swiss Confederation and the Dutch Republic, Tudor-Stuart Britain (remember the Glorious Revolution was still within living memory, if just, in the middle of the 18th century), etc.