I don't think it's about "just" or "unjust" centralization, since (nearly) everyone is opposed to "unjust" centralization and in favor of "just" centralization.
And I do not interpret "decentralization" as a abolition of hierarchies or a complete decentralization to the lowest individual level in all cases.
I interpret it as a bias towards making decisions, allocating resources, and wielding power at the lowest level where something can be effectively managed.
With respect to the US political system someone who prefers decentralization might favor abolishing agencies or laws at the federal level in cases where the states can handle those responsibilities themselves. (And similarly avoiding state-level regulation of anything that can be effectively managed at the municipal level.)