I would assume it's mostly the thicker walls, at least it was this case in Berlin with older buildings staying cool for a long time because it took months to heat the walls. Then those walls were warm in winter and it took months to cool them down (though it didn't work very well in the last very hot summers that were hot early on)
This is still sometimes done. The newest section of the Freie Universität Berlin campus (the “Holzlaube”) was built so that the inside remains comfortable with no A/C and minimal heating. Thick walls, as you said.
I don’t know how applicable architectural cleverness will be in densely populated cities in the developing world that are threatened by global warming. That seems like an impossible situation.
