Longstreet did not such a sentiment during the war, but it is possible that it is a very poorly contextualized allusion to his post-war shift into the Republican camp. I wrote about Longstreet's Republicanism here which you should find relevant, and while it doesn't mention Longstreet, this is about Confederate Emancipation. The one thing I will add there is that, when you get to the part about the Barksdale plan, Longstreet was explicitly in opposotition. Levine, in his book notes that:

Lee's First Corps commander, James Longstreet, clearly telegraphed his doubts about the proposal's wisdom to his subordinates. He believed that "the adoption of such a measure will involve the necessity of abolishing slavery entirely in the future, and that, too, without materially aiding us in the present."