Yes, the aim here was to highlight a period of stability after one of conflict - conflict placed in the past tense here. Sejong says "where once was" (at least in English), and not "where is." Taejong might say "where is now violent division." I envisioned Sejong as a re-builder after discord and challenges to his predecessors' claims, with the focus on moving towards a prosperous and united future in the wake of the conflicts during his predecessors' time. Not his. Compare with Lincoln, who speaks of division in the present tense.
Re: the palace, this is fair. I had been looking at the architecture of Joseon era generally. There are some mild anachronisms elsewhere - Bà Triệu references Hồ Hoàn Kiếm, which postdates her by quite a bit, and wears an áo dài, which postdates her by quite a lot, although these were conscious choices to highlight glories of medieval Vietnam in general, as her time there's less information, and especially non-Vietnamese audiences would just think of the American war there and little else. We weighed whether or not to have Thang Long as her capital (anachronistic) or just Hanoi (VERY anachronistic, but recognizable) and went with the one that might inspire some people to look into medieval Vietnamese history.