A machine leg curl is extremely different, neuromuscular patterning-wise, than a GHR. Being strong at one does not make someone strong at the other one (but I'd guess that strength in a GHR transfers a heck of a lot better to a machine leg curl than the other way around).
Also, if you watch the video from Coach Sommer above, his GHR is a compound movement that incorporates a back extension as well. Far thing from a leg curl.
The movement of a low-bar BS and a DL may look similar (minus the bar placement), but their neurological patterning is very different as well.
About the GHD sit-up, the need to consciously "kick" the legs straight to return to the upright position always seemed contrived to me. Yes it works, but my thought is, shouldn't the well-trained athletic person do it naturally? I've never seen anyone do it right their first rep without coaching.