I don't think it's non-mainstream. It was taught to me in school that the downfall of the Greeks was outsourcing their army to mercenaries and I've heard it again and again in reference to Spartan lore.

It's well known among scholars but I think the title here is aimed at popular culture.

Hoplites (shield, spear, helmet, greeves) were rich citizens which is how they could afford their armor. Many of the men who fought in the Classical era were slaves or foreigners or teenagers, often from Macedonia, called peltasts or psiloi.

…and Carthage famously, no?