It surely would have been interesting if Lincoln had included all slaves anywhere in the United States. I guess we’ll never know what would have happened next. There is a chance that the border states would have won a challenge in the Supreme Court, a very slim chance. There is a better chance that they would have bolted. The House of Representatives certainly would not have impeached Lincoln for freeing slaves anywhere. Lincoln’s political opponents and the Democrat newspapers certainly would have had a field day. He might not have been reelected. The South might have won the war. (Good material for the alternative history writers!) Still, I doubt that Lincoln differentiated between any two parts of the Union (as he understood it) when assessing his constitutional powers. He might have made a case for ending slavery throughout the United States as a wartime necessity. But it would have been a foolish decision politically.
I don't dispute that this is one reason it wasn't done. But it was NOT the legal reasoning.
Lincoln wanted to end slavery. Lost causers want to argue he only wanted to save the Union ("at all costs"). Actually he (and all Republicans) wanted both. And while it is true that they wanted to save the Union more, that fact is too often used to support a claim that Lincoln "didn't want to end slavery," or that the primary cause of the Civil War was not slavery. Neither of those statements is true.