He said "In my defense", justifying laughing at the junior dev because he (the junior dev), had 3 years of experience at Amazon.
That said, I think the interpretation that "he had 3 years experience at a big company, he should have been better" is probably the correct one.
They are so bad that laughing is permissible.
Maybe you just don't spend enough time around arrogant people to interpret this sentence this way :)