It's a subsidy because they're selling at below market rates (see the chart). Exxon doesn't sell "at cost".

It's unclear what the costs in the article describe, but the price difference (>$1/gallon) is much greater than the tax rate (21.9 c/gallon) [1].

The article also says DoD fuel isn't allowed to be sold for non-government flights, regardless of costs.

[1] https://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_State...