What Elon means is, because the longest range Model 3 with 2170 cells will have less than 343 mi EPA range, they don't want to widen the gap between S and 3 by increasing the Model S battery capacity when they switch to 2170 cells. Here is what I mean:
Nov 2016: Model S 100D : 343 mi EPA (with 18650 cells)
Jun 2017: Model S 110D : 377 mi EPA (with 2170 cells) << Cancelled because would make Model 3 look bad.
Jun 2017: Model S 100D : 348 mi EPA (with 2170 cells) << 5 mi range increase because of reduced weight.
Sep 2017: Model 3 55D : 225 mi EPA (with 2170 cells)
Sep 2017: Model 3 80D : 300 mi EPA (with 2170 cells)