This is covered in rule number two --
"2. The lower fare is not offered at delta.com."
Did you not read or did you just expect them to waive it for you?
So explain to me the situation where the credit can be applied? When I booked I saw the price difference existed prior to booking, and went ahead to try for the guarantee. Delta changed the price, was not selling me the same fare class for the right price compared to a 3rd party website at the time of booking.
The price changed, but was out of sync. How does any consumer prove this? Any BFG claim can be avoided by that rule.