No less than is correct because they know the exact number they have sold.
If you’ve sold less than 1,000 units then you have sold a number of units that is under 1,000.
If you have sold fewer than 1,000 units you have not sold 1,000 units but you’re not sure how many units you have sold just that that number didn’t reach 1,000.
But they do know how many units they sold, otherwise they wouldn’t know if they had sold less than 1,000 so it is “less than”.
Fewer than. *
No less than is correct because they know the exact number they have sold.
If you’ve sold less than 1,000 units then you have sold a number of units that is under 1,000.
If you have sold fewer than 1,000 units you have not sold 1,000 units but you’re not sure how many units you have sold just that that number didn’t reach 1,000.
But they do know how many units they sold, otherwise they wouldn’t know if they had sold less than 1,000 so it is “less than”.
No, that’s wrong.
The “less than” vs “fewer than” rule isn’t about whether you know the exact number - it’s about countable vs uncountable nouns.
Video game copies are countable, so it’s traditionally “fewer than 1,000 units” regardless of whether you know the precise number sold.
That said, it’s been misused so much that it’s become more flexible, and “less than” with numbers is widely accepted now.
Don’t take my word for it, Google it.