The British did not pursue the fight. Had they wanted, they could have kept Fort Ticonderoga, and we would not have been able to dislodge them no matter how much we tried.
So a fortification, built by the French and Canadians was seized by conquest by the British. At least once it was captured from the Brits by Americans. Its ownership was calculated by possession, not title.
How again is this even remotely analogous to the seizure of Sumter by the rebels?
Fort Ticondaroga was badly situated for defense against land based forces. It was located to interdict southern Lake Champlain (supplemented with a iron and log chain) near the confluence of the river from Lake George. The British took it with minimal bother (Burgoyne’s expedition) by occupying a lowish mountain nearby. The French defended it successfully once (under Montcalm) by leaving the fort and building field fortifications.
At that time reducing a masonry fortress was a textbook exercise: You selected where you wanted the breach, you dug or blasted parallel trenches that could not be enfiladed until you got close enough to pound a breach in the wall. It wasn’t fast, but it was sure. A relief party could arrive and rescue the garrison of the fort, but it couldn’t hold the fort against a siege. Forts could not be held, but served only to delay. Vauban developed them to guard military stores, and to delay enemy forces while other forces were mobilized and trained.
Ft Sumter was admirably placed to prevent digging parallel trenches. There was no South Carolina soil adjacent to Ft. Sumter upon which parallel trenches could be dug. Still, Ft. Sumter could protect a harbor from attack on the water, but it could not be held against a superior land force.