IMO, the discrepancy reported on the front page is surprising, and probably does have a lot to do with the 'rest' during the sit-up.
I wonder how the ratios would look (for CrossFitters or for other athletes) when strength rather than strength endurance is compared in the two exercises. I've done weighted glute-ham sit-ups, and let me tell you, they're HARDER than back extensions. The range of motion is just so much huger. The back extension actually seems quite comparable to the glute-ham sit-up in difficulty if I only lower my torso to parallel to the ground on the sit-up, pausing there for a moment-- it's getting from hip angle at ~225 degrees to hip angle at 180 degrees that's the killer.
I haven't done the glute-hams for a while, though. Those mess with my back, even with proper extension of the leg.