Frostpunk hits 5 million copies sold in six years, boasting average playtime of 24 hours

I would assume they do. When companies have these free deals, then the intermediary/store hosting the deal will be the party who pays for the claimed copies. As such it is technically copies sold, even though the price is not necessarly the current full price that consumer would pay in another store

Those free copies are paid copies in one form or another.