I find the story a little hard to believe. Why would they bother to calculate mileage by any method other than number of wheel rotations. It would also be very easy to prove - just drive a mile and see how far the car says it's travelled. The fact that no-one is showing this proof suggests the story is made up. One of the selling points of Tesla is how reliable they are - most owners seem to say they cover 100's of thousands of miles and do nothing except change tyres and windscreen wipers.
One possibility might be the wheel size setting for the car might be wrong. If it's set to a 22" wheel size but the car is actually using 18" wheels and the vehicle uses wheel rotations to calculate mileage (like every other vehicle) then a 70 mile run would show up as nearly 90 miles because the wheel is rotating more.
The only thing really wrong with Tesla is Elon.
Except that wheel diameter does not determine this. Tire diameter does. A Model 3 with 18" wheels from the factory has 235/45R18 tires (45mm sidewalls). The factory 20" wheels use 235/35R20 (35mm sidewalls). The circumference difference between these two tires is 0.8%.