My bad you are right I read too fast. They say this about the connector :

UNA uses a standard USB-C cable, meaning less time searching when you occasionally need to charge your watch. Does this USB-C connector affect the waterproof rating? No, it doesn’t as the USB-C connector on the watch is IPX8 rated, meaning the internals of the watch are completely sealed from the outside of the connector.

But I couldn't find where they define their "waterproof rating".