The $5250 rule usually only applies to tuition for employees.
I'm not specifically familiar with the USAA plan for dependents, but the wording is very clear (and typical). USAA is saying that the amount of tuition assistance they provide is going to show up on your W-2 as additional taxable income to you. To ease the burden, USAA is going to add the amount of tax to you pay. You'll have to ask about how they'll do that, but the usual way is to add it to you W-2 and then show it as additional withholding.
