I agree with Winik’s assessment that you quoted. I also agree with Winik’s quote from the London Spectator. Both are correct, IMO. Winik’s continuing words are an assessment of the impact of the Proclamation on the war effort, while the Spectator’s quote is a strictly correct statement about the terms of the Proclamation.
The London Spectator quote implies that Lincoln's proclamation did nothing to further the end of slavery. As Winik points out in his synopsis, that wasn't true. I believe Lincoln knew that.