... I don't consider "decentralization" per se to be a necessary component of libertarianism.
I define libertarianism in terms of the non-aggression principle.
It is entirely possible for a system to be both centralized and not in violation of the non-aggression principle. For example, a bank centralizing all the deposits of its (entirely voluntary and agreeing) customers in one vault, rather than decentralizing those deposits around various locations. Either of those approaches might be better or worse than the other in various ways, but neither approach initiates force.
It is entirely possible for a system to be both decentralized and in violation of the non-aggression principle. For example, a network of terror cells with no central commander or hierarchy, each cell operating on its own in, say, firebombing synagogues or assassinating people whose religion they disagree with.
QED, centralization/decentralization are orthogonal to libertarianism "thinly" defined.
Attempting to posit decentralization per se as a necessary component of libertarianism is an attempt to add either an application thickness or a strategic thickness to the definition of libertarianism.
Which is fine, if you're OK with "thick" libertarianism.
I'm fine with many thick libertarians, because I can agree with them that whatever thing they want to add to libertarianism as a thickness is a good thing, without also having to agree that it's part and parcel of libertarianism.
My "thin" preference has to do with liking parsimonious definitions that allow for strict context-keeping. I'm not into an idea of libertarianism that requires or precludes pineapple on pizza, even though I agree that pineapple on pizza is a terrible and disgusting idea. Absent a component of aggression, trying to cram such matters into the definition of libertarianism just muddies the waters.