Who gets promoted is probably affected more by historical rank, having any command, and a random factor more than having an Old Guard in charge of an HQ.
I always play Germany with the Old Guards and it's very random which Old Guard General gets the autopromotion to FM. Von Manstein, Guderian, Rommel, etc. do not get the autopromotions first, but some Old Guard Generals will get the autopromotions first. Autopromotion is too random with too many factors to focus on just one parameter: it's more complicated than just one factor.