Initially, we didn't think of the use case where a discount would be scoped to a combination of a channel and an account group. But there is a valid case. The simplest example is when Liferay Commerce is being used to manage products and pricing for both a customer portal and a B2B marketplace. If a customer buys from the B2B marketplace, it's likely that the seller doesn't want to offer any discounts to them. But if the same exact customer buys via the customer portal, they can get the discounts they're entitled to.
Update the Discounts mocks to support allowing an admin to make a discount valid in a combination of an account or account group and a channel. It should essentially follow the same pattern as the price list qualifiers.