Let me start by saying that I am not an engineer, but I have driven Tesla's for 11 years
In this video:
They calculated the motor turns over 18,000rpm.
The problem with back EMF is that you need to increase the current the faster the motor goes. Say you have a simple 12V motor spinning and back EMF was 2V. So you had ad applied voltage of 10V. You want the motor to go faster so you increase to 14V. Back EMF increases to 3 so your applied voltage is 11V. Motor sill spin faster, but you had to put in 2V to get 1V applied to the motor.
So in the case of Plaid, I believe there was a battery chemistry change along with improved battery cooling that allows the pack to output more engergy for longer. So I believe they are simply throwing more power at the problem.
Also, I believe this is the first use of permanent magnet motors in Performance S/X. Permanent magnet motors have less back EMF than the AC Async motors that they used to use. But the PM motors are more sensitive to heat (the magnets melt), hence the carbon-sleeved rotors.