Costco must be close to Apple because that is not a retail spec base model - it's the kind of custom spec model that Apple would only allow certain resellers to sell, and they are being sold off cheaply as Apple have technically discontinued the M1 iMac now. They still do the base M1 Max Mac Studio for £1299 (£300 off their usual price) from time to time which I also think is a cracking deal.
I guess it allows Apple to make money from hardware in a price range below that of the latest hardware. If you can live with a year or two less of support overall then these are very decent deals.