It's good, but I think you're unclear on the role of the abs. If the extended lumbar position is established by the erectors on the posterior, the abs and the other circumferential muscles around the spine, ribcage, and pelvis contract isometrically AFTER the lumbar erectors establish extension. There is no tendency for the abs to pull the system into posterior pelvic tilt/lumbar flexion unless you perform this setup incorrectly. And this would be an excellent argument against doing situps if you're physically stupid: if you learn a lumbar flexion in a situp or a crunch, and if you're physically stupid, the tendency might be to allow it to carry over into the squat or pull. But the lumbar extension is easy to teach, and any coach with any experience in this method should be able to correct even a physically stupid person who does this.