If it's true, 13.5 oz isn't crazy heavy. Yes, it's much more than a lot of players use today, but I still don't find it that unbelievable. Her strokes are extremely compact. She's not taking Soderling or Nadal cuts at the ball. So, she can play with a heavier frame not worrying about being late on a massive swing. And, her game is based on redirecting other player's pace, and a heavy frame is perfect for that.
I read once that Sabatini's Prince OS frame was 14.1 oz. Mandlikova's Ultra 2 was around 13 oz. And, think of all those women with bulky wood frames in the past. If you develop your game with a certain type of frame, you'll get used to it. Date is obviously used to the weight. If you have good technique, a heavy frame is doable. It does a lot of the work for you in a way a light frame can't.