I think black upside down triangles are underground cities. Only example of one I see on the maps is Ironfang Deep and that is an underground Dwarf City State/Kingdom. Up pointing white triangles are important mountains. X seems to be miscellaneous important locations.
This is definitely
Mike Schley's work.
Unfortunately, none of the maps that have upside down black triangles have a key, and the ones that have a key don't contain that symbol, so it's hard to know for sure. However, the map of Vaasa uses that symbol for Delhalls, Talagba (both Dwarven mines) and Hillsafar Halls (a Dwarfhold), but also Sorath-Nu-Sum (a former Kua-toa city in the underdark), so it seems likely that it's used for underground settlements. On the other hand, it's not used for the likes of Mithral Hall on the SCAG map.
Upwards black triangle tends to be used for temples.
White triangle is used for mountain peaks, specifically.
Black circle is cities, while white circle is towns, and black star for capital cities.
Slightly frustratingly, Schley is not fully consistent in his use of symbols, where a black X is sometimes used for ruins, and other times for points of interest. In the latter case, either a black square or black rhombus is used for ruins. In the case of the first map, X is used for points of interest (The Dancing Place and The Five Lions) while a black square is used for ruins (Gildenglade was destroyed), as you note.