As far as the optics go of voter choice goes, there is a category of people who because of sovereignty issues will never move from the Bloc, so they are not attainable or reachable for the NDP.
Québec solidaire is proving that a big slice of such a category can in fact be moved.
Then there is another much bigger slice for whom sovereignty is important, but just one among many issues that matter, and the priorities change. And among those people, I rather doubt that it matters right now that the NDP is not talking up the Sherbrooke declaration or its content.
The NDP doesn't need to "talk it up". It needs ideas about how to actually implement it, to make Quebeckers' lives better within the federation. It also needs to ensure that Jack Layton never again does the about-face he did in January 2006.