Yes, unbelief is still a sin for the reprobate.
"The gifts and callings of God are without repentance"
The Creator intended, (by way of a creative call,) for Adam and his race to be godly or god-like, as man was made with the divine intention of holding intimate communion with, and even habitation by his Creator/God, and so man had to be like or similar to God, for:
"Be ye holy, for I am holy". Since then "The gifts and callings of God are without repentance", this creative call or divine purpose isn't something that will ever change, since a perfect God must choose something perfect, and cannot do otherwise. He has no reason to change His mind about any perfect choice of His, and that is why the creative call for Adam and his race to be godly, can't ever be changed by God without casting back a doubt or suspicion upon God's former choice.
This is truly one of the incomprehensible horrors of hell: What a terrible and immeasurable pit of terror and punishment must such a realm be, where throughout all of eternity proper, God's original, creative determination for man to be godly isn't something that will ever change. The reprobate damned's utter inability there to be godly, won't change their responsibility to be the same, nor their liability for disobedience for that matter. What makes eternity in heaven so very wonderful and enduring for the righteous, works inversely for the reprobate in hell.
It has been well said,
"God means what He says, and says what He means."