My understanding is that customers can't buy cushions on their own. At the moment their only option is to add a number of cushions between the cart and the checkout.
But the issue the company has is that not all cushions are the same.
So they were looking to resolve that issue, rather than change it so that people could buy cushions without first buying some furniture.
In theory, as you say, it should be do-able both ways - especially as there are only two sizes of cushion (as far as I know).