I believe the Tankless HWH was standard for the 2015 model year so I would be suprised to find out that HWH volume was included in the capacity and 24 gallons would be a significant drop from 33.
I took thinks apart to get a view, they don't make these things easy. The tank looks pretty purpose build for this model. It is thin and runs most of the width above the axel. It is sloped front to rear with the low point in the rear left corner. There does not appear to be much room to adjust slope anywhere.
I was suprised to find a third overflow hose behind the shield, it does explain why I was getting overflow water coming from locations other then the two hoses I could see with the shield in place.
I will try a few more measured fills and see what I come up with
I might also look to replace that shield with some clear plexi so I can actually view the tank. I hate not having the ability to visually inspect it.