That's ridiculous. Here's a decent way to do it:

1. Desolder the chip (or have a read-only port to plug into) and put it on a test board
2. Read the chip output and send it to the manufacturer (has mileage plus w/ cryptographic signature)
3. Manufacturer sends newly programmed chip to the mechanic, and the mechanic sends the old chip back in the provided packaging
4. Customer gets the car back, and the manufacturer can verify that the chip swap was legit

Boom, under a week turnaround without any backdoors in the odometer.