They are trying to reason that since there is T bucket availability on ATW-ATL and ATL-SRQ individually that DL "should" offer the $75 cheaper T fare for ATW-ATL-SRQ.
And his odds of prevailing are pretty high, especially if the judge takes a common-sense approach to looking at the segment pricing. You & I know how married segments work. That doesn't mean the judge has to buy into such trickery to extort more money from selected passengers.
The plaintiffs were asked to pay more to fly A-B-C when they could purchase tickets for A-B and B-C for less, all the while Delta promises "With our Best Fare Guarantee, you get the best Delta fares or your money back.” A little bit of plain English and common sense in that courtroom could well work in the plaintiffs' favor.