I agree that it is confusing, based on how they have structured points vs. miles earning in the past.
Note also that it is for stays at "participating JW Marriott®, Autograph Collection® Hotels, Renaissance® Hotels, Gaylord Hotels®, and Marriott Hotels®".
List of participating properties: http://joinmarriottrewards.com/united/properties.aspx

The text from the confirmation email is more clear:
"Members can elect to earn miles or points for base stay under this promotion and must make selection in their profile when making reservation. Only miles can be earned for bonus portion of this offer. Valid Marriott Rewards and United MileagePlus account number must be provided at registration. Make sure your preferred airline is United Airlines to earn with this promotion!"

HOWEVER, when making a dummy reservation I do not see a way to specify earning choice; nor do I see a way to specify a "preferred airline" like Hilton does.