The Apple Watch automatically detects that it is not worn when it is worn outside the clothes

That's completely different for the Ultra and what is being shown is the Ocean Dive Computer App for Apple Watch Ultra, specifically for diving. It's also exclusive to the Apple Watch Ultra. Under normal conditions (not diving) the Ultra Watch also will not work on your clothes, without contact to your skin.

For Apple watch to know it's on your wrist, for standard use, the watch must be on the top of your wrist, where the sensors are in contact with your skin in order to work. Outside the wrist on top of clothes, the watch will not know it is being worn.

... which shows that is it technically possible.

I am a semi professional runner, and I rely on an external hart rate monitor and the loss of the built in sensor in the Apple Watch is of no concern for me.

I absolutely think that this should be a feature on the Apple Watch, because not everyone is blessed with perfect weather for outdoor workouts all year long.

I have submitted a feature request to Apple, and I am 100% certain that it will be implemented in the next update of WatchOS! 😉 <- ironic wink.

Not really. The photo you referred to is for a SPECIFIC purpose on the ONLY Apple Watch which is designed to dive with. And that specifically requires using an App, the Ocean App, which is ONLY designed for underwater diving.

This does NOT mean under any other condition, the watch can be used as a watch outside of clothing.

I'm certainly glad you provided a feature request to Apple, but since the sensors on Apple Watch are absolutely required to be touching skin for the watch to function in normal use, your feature request is pretty much guaranteed to be a disappointment 😉

Well, at least for the Apple Watch Ultra, there are pictures showing the Ultra worn outside wetsuit. Even in Apples own promotional pictures.

The watch is not designed to be worn over clothing