In MOTU v Thundercats, LionO barely held up against HeMan who A) didn’t have the Sword of Power and B) wasn’t utilizing the Power of Greyskull to significant degree. HeMan is stupid powerful now, Eternia Wars shows that he can harness the full Power of Greyskull (aka the power of all creation), which is essentially an infallible power. Thundera, while powerful indeed, simply can’t compete.