My best guess about their logic, assuming they didn't just completely misunderstand the systems, is this:

With a chaumian blinding server, you have a situation where none of the N participants know the linkages, and nor does the server. So from a privacy perspective this is decentralized, no one actor is privileged.

However even if you look at it as an academic and not a practicioner, you should see that having a central server coordinating the transaction is a very important centralization, because they can select, and control, which participants are allowed to be in what join event. (*very* relevant for e.g. sybil concerns).

So in short I agree with your question 😄