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. 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.
There are reams of proof that Lincoln did not free slaves in Southern areas under Union control.
Why do you suppose that was?
If his motivation was to free slaves in areas where he had Constitutional authority as military CIC then surely why not free the slaves in "rebellious" areas under Union subjugation?