This text suggests that if you are OW Emerald or Saphire, and in first or biz class, you would have access to the lounge, even when flying within the USA, Canada, Mexico, etc...:

First and Business Class customers who do not hold Emerald or Sapphire tier status are not eligible to access American Airlines lounges when travelling on solely domestic flights within the U.S. or between the U.S. and Canada, Mexico (except Mexico City), the Bahamas, Bermuda and the Caribbean).