The nag 1 transmission has a separate transmission controller so you wont be able to pull transmission codes anyway, any shop with a decent scan tool can pull these codes, but a cheapie OBDII code reader wont, they ONLY talk to the PCM.
Furthermore, what they said is not accurate... it WILL pull codes for up to 40 good trips AFTER the light turns off. When a fault is detected and it turns on the check engine light, the light will only turn off if there are 3 good trips (the computer runs the test on the component or doesn't disagree with the information it is getting from the component 3 consecutive times). After the light turns off it must have 40 good trips before all information on the fault is wiped from the computer....
SUPPRISE, your pimply faced parts slinger isn't a master auto technician
All of that said, if your lights off, don't worry about it, if the light comes on, then worry about it. It may need to go to a shop just to pull the codes unfortunately. The only code you will likely get at autozone if the light comes back on is a P0700 which is simply a code that sets in the PCM to say "hey there is a problem at the transmission controller".