Sounds like they were maybe a wee bit naive.
Just a bit, yes.
James Madison believed (or is commonly interpreted to believe) that partisan politics were inevitable, and that the only solution was to mitigate the effects, not stem the causes. He wrongly assumed a number of things. First, he assumed that only the rabble would be partisans. Second, and therefore following from his first assumption, he assumed that anyone qualified enough to be electable from a party would be smart enough to put the public interest before party. Third, he assumed a multitude of factions, not a bifurcation toward two dominant parties. And on and on. Madison's arguments were not founded in the separation of powers directly, but in the careful analysis of the scale of the prospective government and the proportions of representation. His analysis was reasonably cogent for the 18th century but not for the 21st, and not in light of how the American party system actually developed. But inasmuch as Madison's argument was founded in quantitative electoral arguments, the whole thing exists in the framework of the separation of powers in which those elected to the offices of one power would not necessarily be subject to the same flavor of factious emotions as the other branches.
I heard it say that America is a monarchy with an elected king while Britain is a republic with a hereditary president. Apparently that was said by an American journalist in the late 19th century.
I wouldn't dispute this. I assume entire books have been written comparing the American republic to the British one from which it sprang. But I wouldn't have the time to read them all. I see vestiges in the American system that give homage to a "ruling class," which may or may not ever have existed in valid form anywhere. Judges were appointed and senators given lengthier terms on the basic understanding that the people holding these offices were not the "rabble" of the House, but rather those who -- for lack of a better characterization -- had been groomed into the leadership class. The Founding Fathers had not yet warmed fully to the idea that any of the unwashed rabble would ever be fit to lead. This harks back to the notion of monarchs and peers who followed genealogical lines of succession and were thus bred from youth to assume offices of leadership. (There is, however, a remarkable episode of Netflix's
The Crown in which Elizabeth realizes she has been inadequately prepared intellectually.) Whether some hereditary lord or monarch actually had leadership talents was mitigated in the idea that for better or for worse they would be prepared as well as possible for the role.
I gather the Founding Fathers were somewhat conflicted about the executive. On the one hand it's obvious, having just escaped what they believed to be the tyranny of a monarch, they felt reticent about vesting in one person so much power. But on the other hand, if the executive could be a person who was carefully and soberly chosen and vetted by a similarly conscientious college of electors -- and not so much just the person bearing the proper DNA or having electioneered most viciously -- then they could be less anxious about giving him so much direct power. Thus I can see that the English system benefits from the primacy of Parliament in order to quench the power of whatever random monarch ascends the throne. It makes sense. Succession is what it is, and it points to the next bloke in line regardless of his actual qualifications. That's a rational reaction to having lived for hundreds of years in a monarchy, and taking steps over the following hundreds of years (i.e., since
Magna Carta) to move toward democracy.
In contrast, had the American electoral college worked the way it was in part envisioned, Trump would not have been elected. That was one of the mechanisms envisioned to prevent despots and demagogues from acquiring the kinglike powers the Constitution granted to the executive. Contrary to the feelings that evolved later, the President was, from the start, not to be elected directly or by a purely popular vote. Even in the now-remote case where the House would elect the President it does so by state, contrary to the customary method of polling the House. In the context of your quote, we seem to have elected ourselves into the mess England learned long ago to avoid.