That's wrong according to Jack Rickard, who found that the 18,650 chemistry has been updated too and it has just as high energy density as the 21,700 cells of the Model 3.
18,650 is just a form factor, a "bottle size" dimension, and it would be 100% stupid for Tesla and Panasonic to not use the best, most advanced chemistry is all their cell formats.
21,700 cells have better volume density, which is important for the Model 3 which smaller, but whuch doesn't matter for the 100 kWh battery packs of the S and X.
I.e. S/X updated to 21,700 would likely have a very similar range and power as the current 18,650 based packs.
What a 21,700 refresh on the S/X will allow, sometime later, are 105 kWh, 110 kWh and 115 kWh packs - but those will likely be significantly more expensive than the current 100 kWh packs.
(
@KarenRei made similar arguments too in the past.)