Totally correct. Except that Groundspeak made a change to Letterbox Hybrids recently, in that they must have some "Coordinate component" in them now. So at least part of the letterbox trail now has to have coordinates, a coordinate conversion, projection or such. At least that's what's being enforced by local reviewers in The Netherlands now. Personally I like the alternative ways of getting to the endpoint in older letterbox hybrids better than the now enforced "at least partly coordinates mandatory" guideline.