It wasn't just the E.T. cartridge, they had a bunch of unsold copies of what-Atari-tried-to-pass-off-as-Pac-Man and various other inventory as well - much of which was rendered unmoochable before being buried.
It's a shame everything had to be put back at the end of the day. The 2600 Home Brew scene could have used the Carts and ROM boards!