why would Apple keep such a thing quiet
Because it's a sealed diagnostic port for a non-end-user serviceable product, not a feature. They aren't wishing anything up, it's just not something they have a reason to publicise.
when the Apple Watch's battery-life isn't what most people would consider impressive?
Actually, people's opinions on this are very mixed. Some people are reporting great battery life and improved battery life on their iPhone as well as they switch the screen on less. Some people report the opposite. Chances are, people who have just got a new gadget are playing with it all day, which obviously isn't representative of normal usage patterns or battery life.
Apple's lack of transparency here doesn't much matter, though
Why are you describing lack of publicity about a sealed diagnostic port for non-end-user serviceable goods as a "lack of transparency"? That is bizarre. You wouldn't expect that for any other company, let alone Apple.