Not true at all. Before the EP, slaves were taken as "contraband" or accepted as refugees if they could get to Union lines, but if the owners could get into court the courts would order the "property" returned if owner could demonstrate that the slaves were not providing direct support for the revolution. Without the EP, at the end of the war, owners would be able under the law to reclaim their property. The EP changed that and designated any slave residing in rebelious territory as being automatically a material aid to the enemy and declared them free for all time.
Yes, the EP was a political statement, and a powerful one, but it also had significant real results in permantly freeing millions of slaves.
As I said before, even historians sympathetic to the Northern cause, Bruce Catton for example, do not make that claim. And what about the fact that with the early announcement of the proclamation, several months before the actual issuance, Lincoln implicitely promised that in any states that would return to the Union prior to the January issuance, slaveowners could keep their slaves.
Now, therefore, I, Abraham Lincoln, President of the United States, by virtue of the power in me vested as Commander-In-Chief of the Army and Navy of the United States in time of actual armed rebellion against the authority and government of the United States, and as a fit and necessary war measure for supressing said rebellion, do, on this 1st day of January, A.D. 1863, and in accordance with my purpose so to do, publicly proclaimed for the full period of one hundred days from the first day above mentioned, order and designate as the States and parts of States wherein the people thereof, respectively, are this day in rebellion against the United States the following, to wit:
Arkansas, Texas, Louisiana (except the parishes of St. Bernard, Plaquemines, Jefferson, St. John, St. Charles, St. James, Ascension, Assumption, Terrebone, Lafourche, St. Mary, St. Martin, and Orleans, including the city of New Orleans), Mississippi, Alabama, Florida, Georgia, South Carolina, North Carolina, and Virginia (except the forty-eight counties designated as West Virginia, and also the counties of Berkeley, Accomac, Morthhampton, Elizabeth City, York, Princess Anne, and Norfolk, including the cities of Norfolk and Portsmouth), and which excepted parts are for the present left precisely as if this proclamation were not issued.
The excepted parts are the areas under Union control at the time. The only slaves really emancipated were those who emancipated themselves. President Lincoln exercised no authority in the unoccupied portions of the Confederate States of America.