“For the same reason Lincoln thought the CSA had no right to secede? “
This is not the same.
The southern states were officially in a federation.
Greenland is a separate country legally, under the same ruling family.
Lawyers can always come up with goobledogook justifying why their side can, and the opposing side can't.
There's no honesty in the process. Just trick terminology.
The southern states were officially in a federation.
"Not being able to leave" wasn't how the Confederate states interpreted the Constitution.
Several New England states also threatened to secede before the Civil War, so it wasn't their interpretation either.
I once heard a historian say that, had the southern states known in 1789 that there was no right to secede, they never would have signed on to the Constitution.