I'm not convinced by that interpretation. It does say 'through' Sharpness Docks, not 'from and to .....' the Docks, and any vessel coming up off the Severn through the lock will in reality pass through the Docks en route. It is simply that the tidal waters below the lock are outside of C&RT jurisdiction. The application form is headed TOWAGE REQUIREMENTS: SHARPNESS PORT; GLOUCESTER AND SHARPNESS CANAL AND RIVER SEVERN which makes it clear that they are regarded as separate - it does not simply apply to a journey from the Dock along the G&S Canal and continuing up the Severn Navigation, but would apply to each of the three waters mentioned individually, e.g. a tow confined entirely to the Severn Navigation.
The form talks extensively of commercial towing operation involving multiple tugs, and of a tug and tow with maximum beam 8.5m and maximum airdaft 38m so they're not going to get far with those dimensions.
It's an extremely loose bit of wording though, and is applicable right up to where you come onto the river at Diglis and on up to Stourport-on-Severn.