On one hand why wouldn't they make it 100m? Probably has something to do with it being a chrono and the gaskets.
On the other hand I have read tons of opinions on the topic of water resistance and it is all recycling of the same "you need 100m for swimming". Without any real source or evidence provided that this is indeed the case. If anything I have seen people swim with their dress Pateks or GFs. So there is no reason for me to think that 50m is not 50m, ie more than sufficient.