They’re not referring to the federal road tax , but the $0.009 in the price.
The US actually has a legal denomination that is 1/10 of a cent, called a mill. It’s 1/1000 of a dollar. It’s very rarely used, and was never actually minted. The closest we had were 1/2 cent coins (5 mills), but those were short lived coin denominations in the 1700’s.
So, why do gas stores get to use mills in their prices? I don’t know, but I’m sure they do it either for a legal reason that outdated, because they get to derive extra profit per transaction, or because it’s an extreme form of the ¢99 advertising trick.