Guy,
The thread you linked there was addressing this topic before Pro was released.
Now we have Vendor Coupons so vendors can use coupons for their own products.
If you are using WooCommerce coupons, then they will not calculate properly.
The only time admin coupons can really be used and calculate properly is for admin products.
Otherwise, your vendors can create their own coupons form their Pro Dashboard > coupons.