We don't know if END couldn't do the same. When Natsu was about to change into Demon, he was so strong that he nullified Dimaria's Hax Magic and destroyed her. And then he continued to loose his humanity, growing in strength, to become END. Potentially, END was far stronger than Spriggan's Tier, the same has Grey destroying Invel by rising rage without using Devil Slayer mode. Theses two were at their peak. They almost lost their humanity.
Zeleph didn't impressed me in raw strength. Without his immortality and death wave curse, he was rivaled by Natsu who lost his seeds which previously amped him. Natural Dragon Force is the answer of this lost seed, his climax fire, but as a human (humanoïde) being, not a Demon, not a Dragon. I see Zeleph without Immortality and Curse, at Spriggan's Tier raw strength. Nothing tell us that END was below FH Zeleph as well. Devil Force and Dragon Force were Natsu's climax potential, so the same being, but Seed in control.