I'm not sure all the people who've said it, but I've heard it a lot over the years from ex-coaches, GMs, etc.
I happen to believe it's true, since a defense is always, to some degree, reactive, while and offense is a prepared attack, requiring precision timing, and understanding where the strengths and weakness of a defense are. Those things are about preparation rather than talent. The one big caveat is QB play - if you have incompetence at that position that can limit an offense a lot, but it should be able to be managed since a lot of qbs in this league are pretty bad.