I don't agree with that. The oldest blockchain known has its hash codes published each week in the New York Times. I would agree that having the ability to prove the history hasn't been modified by externally retaining the hashes could be a component, but git qualifies for that as well.
I would recommend reading the Wikipedia article for a blockchain [0]. It's agreed that a blockchain is decentralized and resistant to change from bad actors. If a system doesn't meet those requirements, it's just a fancy database. I don't see how Git or the New York Times example you mentioned are inherently decentralized.