If the gain is REAL then I would assume there would be long lineups of 18 wheelers since any small gain is attractive to them.
Except the fact is pretty much all Exxon stations are geared towards smaller vehicles (car, SUV, light pickup) which is who they direct their marketing towards. I just used an Exxon on Long Island recently - only had one diesel pump well off to the side and is a bit of a chore to get a big rig in/out of there. Was an older pump without any of the marketing hype the OP saw at his station. I refueled there because it was $2.99 compared to $3.15 and up seen at all other stations on Long Island.
Additives are useful for keeping the fuel system clean but not so much for improving fuel economy from what I've seen. After all, that is what they are marketing - a detergent additive. Also possible they used a lesser grade of diesel for the test as that is what the fine print alludes to: "Applies to Mobil and Exxon Diesel-Efficient branded fuel compared to diesel fuel
without detergent additive."
Now if one usually uses additives then stops using same one's fuel economy goes down. One summer long ago I ran out of additive and didn't use any for a few months. (This was back when I had my old A4 Jetta with the tried and true ALH motor.) Well, my fuel economy dropped slowly. A friend gave me a new additive to try and on the first tank I gained 5 mpg! So using additive cleans the fuel system out which is where that gain came from. Of course, using additive on a regular basis one will not see the numbers improve.
So Exxon may be just including an additive in their fuel specifically branded as "Diesel-Efficient" as opposed to the car owner adding additive themselves which many of us do. So yeah, using this 'special' Exxon diesel fuel may be convenient when the weather is bad, extremely hot or cold to minimize one's time standing outside refueling / not messing with additive.