If it is centralised, i.e. one entity controlling write operations, you don't need a blockchain. See e.g. the "when to use a blockchain" flowchart on page 41 (53 of PDF) in the "NIST: Blockchain Technology Overview"[0].

Note also that many systems are sometimes erroneously referred to as using blockchains, for whatever reason (misunderstanding, marketing, etc.). Leaving aside the debate about how decentralised it is, the project you mention has a distributed ledger, but uses a Directed Acyclic Graph (which they call a tangle) rather than a blockchain. Similarly "permissioned blockchains" or "private blockchains" aren't really blockchains in the strict sense because they don't need a consensus mechanism (indeed many now call themselves Distributed Ledger Technology systems).

[0] https://nvlpubs.nist.gov/nistpubs/ir/2018/NIST.IR.8202.pdf