To not be sneezed at means to not be something that should be ignored or passed over, and this context suggests that a free meal is something not to be ignored.
Consider the statement "A lot of them were here, because a free meal is something that should be not be ignored.". Based on this "A lot of them were here, because a free meal is not to be sneezed at." a true statement? Yes, no, or I don't know?