You're right, sensor size is not actually listed. My interpretation is that "Image height" is not the same as the diameter of the image circle. The height of a 1/3" sensor (3.6mm) falls near the middle of the 3.49-3.88mm range. A 1/6" sensor has a diagonal of 3.0mm, but I think the wide end of the focal range makes more sense for a 1/3" sensor.
Just some comparisons: The HF G10 has a 1/3" sensor and starts at 4.25mm. The HF R21 has a 1/4.85" sensor and starts at 3.0mm. The FS40 has a 1/6" sensor and starts at 2.6mm. The new lens starts at 4.36mm.