Hi fantam,
About your first point: the open world demo collection.
It is one that is multiple years old by now. even though it appears to “work” in ue5.
I am not 100% on what is enabled in the project by default and what is enabled by ue5 as standard. there might be some conflict/difference in the two. An explanation might be as you stated that it might be compiling certain shader’s only when the object itself is used. like sometimes when you have a new project you open a certain folder with shader’s and it suddenly starts to compile them. something similar might be happening here, but then with allot of different objects/meshes within the same folder. also perhaps “distance fields” being enabled might compile all those for all the meshes in the project. all those things could cause quite a long processing time 
for your second point: What i think they are trying to suggest is instead of using simple static meshes as foliage you can also use foliage actors. The difference being meshes vs actors. On a actor you could add additional blueprint logic. like “gather fruit”, “chop down tree”, play falling leaves fx etc. There are better way’s of doing these things now than there were at the time of that tutorial. because you should never have a complete forest be actors because of performance reasons. we used to simply replace “instances” with actors according to distance to player or interaction from player. hopefully this explains your questions a bit.
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