Possible Grammar Error

I asked a friend of mine to read something (on skype), and he declined the offer.

Then he asked me to play a game, to which I replied, "I have better things to do."

Then he wrote back, "Exactly! Why I didn't read."

Then we got into a debate about whether it was grammatically correct.

I believe that if he had said "Exactly why I didn't read your text", it would have been correct or even something like "Exactly why I didn't read", but he says he is correct.

Can you tell me whether he is mistaken or I am?