After finding a rate at a hotel I'm familiar with, through the link, I logged in through the standard spg.com and searched the same property/date. It came back with the same "SPG Member Exclusive Rate ". So it appears there's no need to starting using a new booking channel for these rates.
EDIT: Be sure to read the details. It was 10% off the Prepaid rate and had the same prepaid terms - "The time for canceling without penalty has passed. If you cancel, the forfeiture amount will be 100 percent. "
I saw no discount off the flexible rate.