On Jan. 1, 1863, U.S. President Abraham Lincoln declared free all slaves residing in territory in rebellion against the federal government. This Emancipation Proclamation actually freed few people. It did not apply to slaves in border states fighting on the Union side; nor did it affect slaves in southern areas already under Union control. Naturally, the states in rebellion did not act on Lincoln's order. But the proclamation did show Americans-- and the world--that the civil war was now being fought to end slavery. Lincoln had been reluctant to come to this position. A believer in white supremacy, he initially viewed the war only in terms of preserving the Union. As pressure for abolition mounted in Congress and the country, however, Lincoln became more sympathetic to the idea. On Sept. 22, 1862, he issued a preliminary proclamation announcing that emancipation would become effective on Jan. 1, 1863, in those states still in rebellion. Although the Emancipation Proclamation did not end slavery in America--this was achieved by the passage of the 13TH Amendment to the Constitution on Dec. 18, 1865--it did make that accomplishment a basic war goal and a virtual certainty.
The above italicized is from a Lincoln admirer and presents some flaws in your argument that Lincoln only acted "constitutionally". Lincoln acted pragmatically, strategically and politically. He freed the slaves when and where he did simply beacuse he wished to, to sate the Radicals, and most importantly to try to damage the infrastructure of the South at which he was at war.
The idea that the EP was a political, pragmatic move doesn't invalidate that it was done in a legal manner.
Moreover, your source is not a statement of fact, but an analysis by somebody. That analysis is based on the opinions of the author and is not a valid proof.