This subject is confusing some folks. I have been through the process earlier this month. I was able to purchase the miles on offer at the time of booking my flight. You "purchase' the miles at a discount to the usual buy/transfer miles methods. The miles are credited instantaneously. Cancelling the flight thereafter is up to you (the cost of doing so depends on fare basis), but you will retain the miles (as happened to me as I had to return home for a family emergency before using the flight in question).

I could not however purchase miles on a flight that I had booked prior to this offer being released. I tried but could not find a way to do it.