A: ((All friendly Tempestus Scion operatives and all friendly Guardsman operatives) within 6" of a friendly Guardsman sergeant) are issued that order.
or
B: (All friendly Tempestus Scion operatives) and (all friendly Guardsman operatives within 6" of a friendly Guardsman sergeant) are issued that order.
In case A, as you say, an all-Scion force would be literally incapable of ever being given orders (and would have no playable Strategic Ploys and a leader with a useless special ability), while in case B, Scions would always get them and Guardsmen would get them if they're close to a sergeant. Personally I think the combination of the fact that reading A means your elite operatives can't receive orders / renders the Tempestor's ability useless, and the careful repetition of "all friendly operatives" (i.e., it's not "all friendly Imperial Guard operatives within 6" of a friendly Guardsman sergeant") shows pretty clearly what the intent is. Could've been written more clearly ("All friendly Guardsman operatives within 6" of a friendly Guardsman sergeant and all friendly Tempestus Scion operatives"), but I think it's pretty fair in an ambiguous case to take the reading that makes sense.
