Suffice to say that Xemnas is not all-powerful. He does not possess universal power (not even worth mentioning the fact he didn't absorb a complete KH), although he is the most powerful being at that point in Sora's time within the KHverse which the FF characters present therein could not beat (apparently, even while banded together). Within this KHverse, actually "defeating" darkness requires one to be wielding a Keyblade, a weapon which only four characters possess (if you include Kairi), this not representing the perpetual strength of the original KH characters (or lack thereof in FF characters represented therein) but instead denoting the power within their hearts which marked them to be chosen by the power of the Keyblade.