One Man Army really doesn't make any sense right now functionally. Don't know why it doesn't just measure party size. However, it does kind of make sense to be able to choose it at 8 for lore reasons. Fairly sizeable spoilers as explanation:
When you go to the hall of echoes for the first time, your God tells you that you're the chosen one (Godwoken) and that you should abandon your companions. Given level 8 is around when you leave the island, it could be a sort of defining moment to choose to abandon your companions at that point. Somehow you gain power from accepting your fate as Godwoken and going solo. This makes slightly more sense than being able to start out as an uber soldier right from level 1.
Regardless of the explanation, being able to start as one man army at level 1 would be nice just as a playstyle choice.
