It wasn't even a 707, it was a prototype model 367-80.
There were some unusually significant structural differences between the prototype dash 80 and production 707's, for example:
> The 132 in (3,400 mm) wide fuselage of the Dash 80 was large enough for four-abreast (two-plus-two) seating like the Stratocruiser. Answering customers' demands and under Douglas competition, Boeing soon realized this would not provide a viable payload, so it widened the fuselage to 144 in (3,660 mm) to allow five-abreast seating and use of the KC-135's tooling.
It's in the wikipedia article, but just want to call something out: That particular airframe 367-80 is in the Air and Space Udvar-Hazy museum at Dulles. It is gorgeous.
Those two pages disagree. Your quote is from https://en.wikipedia.org/wiki/Boeing_707, but https://en.wikipedia.org/wiki/Boeing_367-80 has "The Dash 80 fuselage was wide enough at 132 inches (335 cm) for five-abreast seating; two on one side of the aisle and three on the other. The fuselage diameter for the production KC-135 was widened to 144 inches (366 cm) and Boeing originally hoped to build the 707 fuselage with that width. By the time the Boeing company committed to production, the decision had been made to design the production model 707 as a six-abreast design"
The aisle width and/or chair clearance probably changed as well. Indeed, if only 26 cm was required to add another seat to each row, there must have been substantial wasted space with four-abreast seating.
There were some unusually significant structural differences between the prototype dash 80 and production 707's, for example:
> The 132 in (3,400 mm) wide fuselage of the Dash 80 was large enough for four-abreast (two-plus-two) seating like the Stratocruiser. Answering customers' demands and under Douglas competition, Boeing soon realized this would not provide a viable payload, so it widened the fuselage to 144 in (3,660 mm) to allow five-abreast seating and use of the KC-135's tooling.
https://en.wikipedia.org/wiki/Boeing_367-80
https://en.wikipedia.org/wiki/Boeing_707