I sent them a message on Facebook asking if to complete the 'task' you needed to actually get all 15,000 points or if you only need to participate in the double points promotion by staying 2 nights. this is what I got back:
"Joel here, P*******, on behalf of Frank. Yes, you need to receive 15,000 bonus points in order to accomplish the "Double Your Stay" task. Once again, double points will be awarded on all qualifying stays starting on your second qualifying stay (maximum bonus award is 15,000 points). Let me know if you have any questions."
So, indeed, it appears that $1500 are required to accomplish that task (as part of the 4 of 4, or whatever, to get the bonus points).