I just read about this in the Washington Post (Marriott's HQ is in neighboring Bethesda MD).
I think Marriott is trying to milk profit out of any and all sectors of vacation rental markets. Air BnB must have looked so enticing to Marriott, they had to do it on their own. They might be shooting the larger timeshare resorts in the foot...and I'm not sure how many more they're building from scratch, either. But in a way they'll be competing with themselves...folks have only so much $ for vacations and so little time. And families are shrinking, at least in the USA, and generally folks don't need 3+ bedroom lodging options.
My prediction though...if this turns out to be a success for Marriott...and it will take a few years for them to see the numbers...they will make a bid to take over Air BnB, buy them out or merge with them. Only time will tell, but the Marriott brand is always pushing for more market share, more profits, more visibility. (Why do I have a PacMan image in my mind?)