They are, and it's been their policy to do so since Day 1, but they haven't enforced it widely.

edit: Clearer: Apps that work with other watch platforms are now in conflict with Apple's policy of not mentioning a competing platform, because now Apple has a watch platform. It makes sense from an Android-iOS sense, because the systems aren't compatible, so no real need to mention it. But if the entire purpose of your app is to connect an iPhone to a watch, your option is the App Store.

What will the developers put in the description? "This is an app that connects your iPhone to...well, Apple won't let us tell you! Check out our website to learn more about our app."

Apple should update their policies and implementation of such to specify this as "mobile phone platforms".

No, their policy is to reject apps that have an app store description mentioning competing platforms.

So you can integrate with other devices, as long as you don't tell your customers in your primary sales channel about it...

Apple quickly reversed that rejection, and stated that it wasn't their policy in regards to Pebble.