To me, it's not like either situation really. Both those situations (if a team goes back to Seattle) involve a team moving to a new city, and a few years later, an expansion team coming in. In this Celtics/Braves/Clippers thing, it was just owners switching spots and a big trade. The T Wolves aren't the Celtics because their best player came to Boston and 6 of our players went to Minnesota. And if Wyc and Mark Cuban decided to switch spots, Dallas wouldn't become the Celtics. The history of the team is not the owners. Now, if the Celtics had moved to San Diego and the Braves moved to Boston, yes, the franchises would have changed and the Clippers would be the old Celtics. And arguably, if the entire rosters had switched places, it would be the same situation. But the Celtics franchise and most of its players (and most importantly, its GM Red Auerbach) remained - only the owners and some players changed places.
The new Browns are not the old Browns and any new Sonics team will not be the old Sonics team. Even if the league placed a new Sonics team in Seattle now, it wouldn't be the same thing, because the entire roster would be gone. Now, if the Sonics got to keep Durant, then I'd say they're still the Sonics. But when a team takes part in an expansion draft, it's an expansion team, and not an extension of a previous team.