I'd say distributed is about redundancy and scale within a single entity's infrastructure, where as decentralized implies multiple entities working together.
This started me thinking about counterexamples, and I thought of DNS. And it occurred to me that it's actually a good way to phrase the argument against split horizon DNS: DNS was designed to be a distributed system (eg like DB replicas) and split horizon DNS abruptly changes one branch of it to a decentralized system (eg like Git) without updating its users.