According to "Appraisal of Injuries and Occupational Diseases of Workers" and "Orthopedics, Plastic Surgery and Burns", if the interphalangeal joint of one thumb is severed, the disability level can be determined as grade 7.

Right, because the thumb plays a more prominent role in the hand than other fingers. So the disability level of thumb is higher than that of other hands. For example, the index finger, middle finger, ring finger and little finger are missing a joint, and the disability level is 10, but the thumb is missing a joint, which should be rated as level 7. It can be seen that the part where Zhao lost his thumb should belong to disability level 7.