I imagine it’s possible they didn’t include the proper driver to support the stylus yet. Perhaps someone with the x16 can find the driver and try to copy it over?
From Chris’ photo, it is clear that the iwork 10 uses the same GT9110P controller as the x16 pro which supports a stylus. The GT9110P datasheet shows that it can support 6 different sensor modules (hardware selectable depending on the connection of the Sensor_OPT 1 and Sensor_OPT 2 pins). So my conclusion is that the x16 pro and iwork 10 Ultimate use different panel and sensors so the stylus are not compatible.