I'm pretty sure that for static stability (CG in front of the neutral point), an UNSWEPT flying wing needs to have some effective reflex in order to trim for level flight. Once you go to a delta, or add enough sweep, the connection between stability and need for reflex goes away (there's a reason you don't see many unswept flying wings). I suppose this is one of those myths where something that applies to a special case (usnwept flying wings) gets communicated as a general truth.