Wow! You are 100% correct. I just watched that scene and the H doormat is clearly visible. I wondered if maybe it could have been explained away by his wife or son's names, but they're Rachel and Teddy, so it's definitely a mistake.
Interestingly, in season 3, episode 17 (or 18 if you go by Wikipedia - it's called "Cocktails"), Michael and Dwight visit David Wallace's house again and the mat is different. The front of the house looks different, too (the columns and the front door have definitely changed).
Thank you for asking this question! I'm going to add this goof to my article.