That's a question for Shinji whether he can sustain 9 strokes in 20m (25m -5m push) for longer distances. In the 9 stroke video he's swimming at 1.8 sec/stroke tempo. That translates to a 1:21/100m pace (without walls), e.g. 9 x 1.8 x 5 = 1:21 100m pace.
His 12 stroke viral video, Shinji's swimming at 1.35 sec/stroke tempo that translates to 1:21 100m pace, e.g. 12 x 1.35 x 5 = 1:21. Both (9 & 12 SPL) put him roughly at a 20 min 1500m swim provided he could sustain that tempo and maintain stroke length.
I believe Shinji's open water 'sustainable' SPL and tempo is 15 SPL at 1.0 sec/stroke tempo; that translates to 18:45 1500m, e.g. 15 x 1.0 x 5 = 1:15 100m pace (x 15 = 18:45).
Regardless of the athlete, swimmer, runner, cyclist, hockey, basketball, soccer, etc - mastery makes it looks so easy, movement economy. This is the toughest perception to shift to for swimmers trained in conventional, popular methods of more yards, harder sets, faster turnover can only make you faster.
I love math, physics AND swimming :-)