I think you are reading his post wrong. Best Buy is $250 off the device. But it is $250 off over a span of 24 months. You have to sign up for the 24 month payments to get $250 off. So technically, it's around $10.41 off your bill every month.
What Marlin said is that you can stack this deal with the double trade in special that Samsung is running right now. So if you trade in a Samsung Galaxy S8+ (for example, you'll get $450 of trade in value. That stacked with the $250 off will make the Note 9 really only $300 if you do the 24 month payments.
Does that make sense? I know it's a lot of info.