Sounds an complement to the continuous Buy/Gift miles promos..
This specific reduction is attractive but limited. $20 per groups of 1,000 Skywards miles. Without previous promos, a group of 10k miles would cost you $300 (if not discounted) , hence a good discount 33% for 1k ($20 vs $30).

I have booked a ticket today after have received the promo email and the link takes you to the MY TRIPS.. under the specific one there is no way to multiply this trip miles. So have to check later..

Anyhow all in all, sounds a fair and cheaper option to add more miles - baseline is limited to the total miles one would get on that specific flight.