> If you can afford to pay an army you can afford to pay the opposing side instead.

Aside from all questions about how such an agreement is to be enforced once you no longer have the money but the invaders still have their weapons, the article shows very clearly that this is not true. Early states are seriously cash-strapped, and rarely pay their armies in easily portable goods. They can "afford" to raise armies consisting of soldiers who bring their own weapons and, by-and-large, their own food. That does not make for good tribute and so, in fact, they cannot afford to pay off an invader.