The only reason this might not be so big a deal is if the oath he took is substantially identical to the one required. I'm not sure what oath he did take, but if it's just misplacing the "faithfully" without changing the fact that it modifes "execute", then it certainly sounds very similar.
People keep missing my point. It's not about the oath. It's about him making a decision to forgo doing the right thing. He looked at the situation and said "Well, I know he said told me to say the oath incorrectly. What do I do? Ah, heck, I'll just say it wrong too. Seem like the best option right now."
Get it? He didn't do the right thing. He did the expedient thing.