Just called the Marriott Rewards main number to ask for Platinum challenege and was given it with a verbal that I am Platinum with IGH Priority Club. 15 seconds later, I was given the challenge with only a 9 stay requirement, not 18. I checked to make sure this was right and that I wasn't being signed up for the "gold" challenge and she confirmed it was the Platinum challenege.
Not sure if this was a fluke for me or if Marriott is reducing their requirement????