Where do you get that idea? Is everything in your world black and white?
How can the UK be a fully sovereign nation if it can't control its own borders, its own waters, its own law making, its own judiciary, its own trade policy, its own agricultural policy? Please explain.
The UK made concessions in order to join the single market. They can choose, as they are doing, to end this deal and the concessions which are part of it. This would suggest to me that they are sovereign. They have the ultimate say on what happens to their country.
Massachusetts can not choose to exit the USA, it is not Sovereign. The USA can choose to exit NAFTA. It is sovereign.
Every country makes concessions to one extent or another in the name of global trade. It is a question over who has the ultimate control over these concessions. In the case of the UK, it's clear that they have the ultimate say and there fore are sovereign.
If the EU as a block, forbid them to end the deal and leave, they would not be sovereign.