The dimensions are included in the name:
- The 18:65.0 cells have a diameter of "18mm" and "65.0mm" height.
- The 21:70.0 cells are thicker as well the "21mm" is diameter and the "70.0mm" is height.
So given that the 2170 cells are not just thicker, but also denser, and the copper cooling channels with glycol circulating are wrapped around the cells tightly in the current Model S/X battery module design:
For this reason the battery modules probably need significant rework as well: the cooling channels probably need to become a bit wider, to be able to transport more heat away from the denser cells.
I believe this is why JB mentioned that both the modules and the pack need rework. I don't think it's just a 'cell drop-in' - the diameter and energy density differences force a different module design.
But the biggest deal is to be able to fit the 5mm taller 2170 cells into the existing S/X battery pack's vertical dimension, which would leave the rest of the car mostly unmodified.