No, it's not.
What it is is a predicate statement.
If a person had never heard of God does not mean a person has never heard of God. Romans 1 makes it clear that is not possible, but that fact is irrelevant to the argument being asserted. The argument was predicated on a hypothetical, a hypothetical that has no basis in reality but a hypothetical, nonetheless.
If I'd never breathed air, then I could breathe underwater.
If we call a sheep's tail a leg, then a sheep has five legs.
If I drive fast enough with an oscillating over-thruster, then I can drive through solid matter because there is much more empty space in a rock then there is matter.
If a person had never heard of God, then His existence could not be affirmed.
If a person has never heard of Africa, then its existence could not be affirmed.
If a person had never heard of Josheb (or
@Hazelelponi), then his (or her) existence could not be affirmed.
If a person has never heard of quazjarmnaboklschmarge, then its existence cannot be affirmed.
It has absolutely nothing to do with general revelation and everything to do with the basics of logic.