Hey there knarfy,
We understand you couldn't get Siri to work on your Apple Watch Ultra. We'd be happy to provide information to help you out.
This feature has not yet been released. It will be released later this year as mentioned in the footnotes here:
Apple unveils Apple Watch Ultra 2 - Apple
"On-Device Siri with Ability to Access and Log Health Data
For the first time on Apple Watch, Siri requests can be processed on device. For requests that do not require information from the internet, like starting a workout or setting a timer, Siri does not rely on Wi-Fi or cellular networks, resulting in quicker and more reliable responses. The powerful Neural Engine also makes dictation up to 25 percent more accurate.
On-device processing is private and secure, and now Siri can be used to access data from the Health app for health- and fitness-related queries. For example, a user can ask how many hours of sleep they had the previous night, for progress on closing their Activity rings, to start an outdoor run or cycling workout, or make requests like setting a waypoint with just their voice, even with no connectivity. Users can also make Siri requests on Apple Watch Ultra 2 to log health data such as their weight, body temperature, period, or medications taken."
Kind regards.