yeah, good point. when i wrote that guide it seemed to work, but it was actually chance. i tried to consistently choose which soldier it sends out, but without much success. I've noticed that among all the soldiers in a fortress, the computer is most likely to send out the last one (in the order you see them when opening the building). Also, most of the time the soldier in the first slot will be the one to stay behind. But those are not 100% guarantee.
Furthemore, clicking soldiers out doesn't always result in the soldier i want occupying that position; sometimes it rearrange them in strange and unpredictable ways. My previous belief that the level 10 will go out first came from a game where the level 10 was in the last slot, it went out, it killed a few enemies, then it went back and got back in the last slot. But it was chance, and not very reproduciblle.
The situation is even more compliated if there are several buildings that can send soldiers: in that case, the program normally send a soldier from the farthest building first, but again, that's not a guarantee.
Rigth now, I don't have a reliable solution. I either wait to have two level 10 soldiers, puut them in the same building, then kick out all the other soldiers so that I am sure one level 10 soldier will go out, or i attack with many soldiers and accept the losses of a few unpromoted soldiers, confident that the level 10 will let me kill more than i lose. depends on situation. But yes, I don't know a reliable way to always get out the level 10.
I suppose I should update the manual and then ask einstein to update the page, but I'm too lazy to do it. it's not a big deal anyway, everything else works fine.