It can be misleading to calculate things this way. Add 3 turns, + DPS loss throughout the 100m (especially if he's maxed out on the first 25). Most swimmers shine their best on a single 25m totally fresh knowing the the video stops after this 25m.
You know, demonstrating the potential in regard to lowest stroke count contest, and swimming one's fastest are 2 entirely different things. Shinji has mentioned a few times, over the last decade, that he was interested into racing at some point (most recently he shown interest toward an AUS OW race, don't remember the distance). Only when he does, will we know how fast he can swim (because then he is going to have to aim for a trade off between SPL and SL). For now, that tad bit above 19sec was beaten by Andy in Norways who reported having swam 18.some on a 10stroke diet. That again, with a few seconds per 100m lost in turns, translates into a critical pace of 1:20/100m, 2:40, 5:20, 10flat, 20flat, given that this SL/SR combination is sustainable over 1500m.