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”.