That’s exactly what I meant 
No, not all all sure - that’s why I was also talking about building blocks (or “elements”), as opposed to “entire buildings”. E.g. certain types of roofs could be instanced, or windows, or… but even that is just an educated guess at best.
I was just saying that generated geometry could profit of instancing (as opposed to “phogogrammetrically generated mesh data”). But I am pretty sure that Asobo is using instancing somewhere, e.g. cars (or even “generic aircraft” come to mind). And of course “grass” and “trees” that you’ve mentioned.
My answer was hence just an attempt to answer the question of the OP why generated (“artificial”) mesh data has certain performance advantages (“you control the data”), but also disadvantages (“not as detailed / good looking as photogrammetry”). Respectively in the end it is also simply a question of “who (how many artists) is going to model each and every individual house” vs “let the computer / algorithm determine the 3d objects” (= photogrammetry).
But we can all look forward what else might come to light, with further “AI generated environments” improvements (and a larger library of “building blocks”, more refined “building rules” (“no trees right in front of runways” ;)) etc.
Btw. a friend I know from my studies founded the company Procedural, with their product CityEngine (Procedural is now part of Esri): CityEngine is basically a way to automatically create (artifical or “real looking”) streets and entire cities, based on such “building blocks” (“primitives” such as doors, roofs, chimneys, walls, and all those architectural “style elements”, …) and especially “rules” (e.g. a three stories building must have a chimney", “in this area the buildings are of commercial type” etc. and what not):
CityEngine was (is) also used in various Hollywood movies (in preproduction, but also in some actually generated visual effects in the final movie). 
Probably. And cars quite possibly.
UPDATE: For those who are interested in this “rule (procedural) based modelling”:
(“the basics” start at the 00:8:15 timemark)