I also share your confusion: if a central body is trusted in this scenario, then why can't the atomic transactions be equivalently performed on a public Merkel tree with a centralized source of truth?
Disclaimer: I am not a lawyer and the previous post is my understanding of the described as a layman. I am pretty firm on the technical concepts surrounding blockchain solutions, though.
Compliance with present and future privacy laws could easily become an issue, though, unless those laws contain specific carve-outs.
(I'm also no lawyer, I've just been interested in the cryptography surrounding the big blockchain projects. Much of it looks like it could be surprisingly useful outside the trustless, decentralized contexts everyone likes to talk about.)