I think you guys are trying to turn Romo's words into something they weren't intended. Garrett is the head coach who isn't calling the offense or defense, so, like Jimmy Johnson, his role has nothing to do with being a schematics guy. That doesn't mean he doesn't understand schemes. The guy was successful in the NFL as a QB with limited physical ability and won some huge games when called upon against top competitors; he understands schemes. He started as an offensive coordinator, and he called plays and was successful doing it in Dallas, despite a lack of top talent at times, and he gave it up after Jerry realized that as a young, first time head coach, he was making mistakes managing the game. His play-calling wasn't the issue; his game management decisions were, so he had to step back and get that right.
Now, he may not be a great head coach; he may never get the Cowboys to a SB or NFC championship victory, but it's foolish to suggest he doesn't understand schemes. That's not at all what Romo said. Again, like Jimmy Johnson, that's not his role, so he's not doing it as head coach.