At first I thought excluding mint was kind of disingenuous, but then I read the initial e-mail again (highlights mine):
I had initially interpreted "an" as "any".
Redeem your TrueBlue points for an Award Flight by August 4 and save 25% when you travel between September 7 and November 16, 2016!
To take off now and save points for later, simply:
Head to jetblue.com, where all eligible Award Flights fares will automatically include the 25% discount.
Book between now and August 4th
Fly September 7 November 16, 2016 (Blackout dates 10/6 10/16/16)
Fly Mon Thurs and Sat. (not valid for flights on Fri. or Sun.)