Idk if git was designed to not be used in a centralized way. Like all other common CLIs, it was simply designed to work on your PC or server without caring who you are, and nothing stopped a corp from turning it into a product. Torvalds made git and Linux, then he put Linux on Github.
The reason Linus wrote Git was specifically because he was unwilling to accept the centralization of the existing popular source-control systems like CVS and SVN, and Linux's license to the unpopular proprietary decentralized source control system it was using got revoked because Larry McVoy threw a tantrum. Linus needed an open-source alternative, so he tried the unpopular open-source source-control systems like Monotone, but he felt they suffered from both featuritis and inadequate performance, so he wrote a "stupid content tracker" called Git.
"Distributed" was the point and the language their site uses*, not decentralized. It's only described as a convenience and reliability thing in contrast to the mess known as CVS. I haven't seen a note about avoiding one entity having too much power, even if that's a goal some users have in mind. Normally you have one master repo, or "blessed" as the site calls it.
It's like, a Redis cluster is distributed but not decentralized. The ssh protocol is not decentralized. XMPP, Matrix, and Bitcoin are decentralized protocols, first two via federation.
