We've seen different Starbrands exhibit different levels of power, true enough, but everything we've seen suggests that's down to the limitations of the individual wielders' imagination, not the potential power level of the Starbrand power source itself. From the 80s Starbrand on, that's how it was presented... and note that with the 80s Starbrand, Ereshkigal was able to do far more with it than its original user in the New Universe.
In light of that factor, it doesn't seem like much of a stretch that the current/deceased Starbrand might be only megapowerful under normal circumstances, but be able to tap into more of the power's potential when dying and facing Beyonders. Indeed, if the limitation is the wielder's imagination, then being able to take out one's current opponent doesn't actually take a lot of imagining, does it?