He was talking about spawning objects while the game is running.
Actor spawned from within editor plugin will stay on the map, well, unless you forget to save the map. It is the same thing as adding them manually.
Actor spawned while the game is running will disappear when you reload the level.
However, you could store list of actors to spawn in external file/resource and implement their persistence this way.
Potential issues:
Last time I checked, you can’t bake lightmaps into objects you spawn at runtime. No such problem when spawning stuff from editor plugin.
Also, when spawning large number of objects into editor (5000 of them) I encountered minor issue where you’ll encounter significant fps drop (down to single digit framerates) right after spawning objects, and fps loss will stay until you press “build” button (meaning it is instantly fixable). After that fps goes back to sane level (60…70). I’d guess you’ll get the same issue if you start spawning lots of objects at runtime, except that there won’t be a “build” button to press.