I don't know why Chopard decided to design the watch that way, but there are plenty of true-GMT watches with lots of water resistance--some even include chronographs that can be used at depth (JLC produces both GMT and chronographs and GMT/chronos with 1000m of WR).
It is much more likely that they didn't want to put the work into a proper GMT, with an independent hour hand and 24h indicator. Adding just the 24h hand is much, much simpler. IMHO, it's not useful, although I otherwise love the look of the watch. It also cannot be a diving watch any longer because it lacks the timing bezel.
Cheers,
Daniel