He means a widebody with 200-250 pax would only have a few seats more than A322 at the cost of a considerably heavier weight. CASM wouldn't look so attractive.
2-3-2 is just too difficult to be justified when compared to a 3-3 configuration IMO. Why add another isle for the sake of gaining one seat only? I know 797 would be implementing newer design techniques that would make it highly efficient. But why wouldn't Boeing apply the same advanced methods to get an even more efficient 3-3 MOM?
If Boeing goes cleansheet will be a tight 8 abreast for sure. The more seats you have the greater the CASM advantage. Its the only way to get the CASM down low enough to smash the narrow body aircraft. 7 abreast would struggle to match the A321 as you pointed out. A 6 abreast MOM wouldn't offer much extra capacity above the A321, if it had extra range(weight) added then it would probably have similar CASM on short trips.
The dimensions and weights of a cleansheet MOM would be within a couple percent of an A300. The A300 is a perfect example showing just how light an 8 abreast widebody can be if optimised for short range.
With composites and new engines the MOM should be lighter and fly further than the 40 year old A300. An A300 size/weight MOM with modern engines would need only 70% of the A300's fuel capacity to fly 4000nm. The max takeoff would be coming down close to 150T. That is really light. About 80% of the seats of the 787-8 but much lighter.
I'd assume two fuselage lengths, with the shorter aircraft having more range.
I still think Boeing will go with a fully optimised lightweight 787 built for short range efficiency. The 787-8 will then get cancelled with free upgrades to the larger long range 787-9 or the efficient short range lightweight version. It would explain the increased rate of production.