The HIG is a set of guidelines, not strict rules. Third-party developers cannot change what happens when you Force Touch the display as it is confined by WatchKit to the typical menu but some of Apple’s apps already violate the published documentation. For the most part, the Apple Watch UI is very consistent but there are exceptions.
For example, if you Force Touch on the Watch display when selecting an animated emoji in Messages, it changes the colour of the emoji. Similarly, Force Touch on the clock face to bring up a gallery of alternate faces. This is arguably still a ‘menu’ but it is very different to the circular button overlay menu as described by the HIG. These special cases are not addressed in the guidelines, unfortunately. It will be interesting to see whether the native SDK (due sometime later in the year) will include the ability to override the default Force Touch behaviour.