I don't even understand what an "unmodifiable blockchain" held by a single private entity could mean.

They

- have the data of yesterday

- have backups of the data of yesterday

- are going to write today's data on top of it to continue the chain

(insert magic here)

- somehow we are super sure, tomorrow, that today's data has not been tampered with

If somebody can shine some light on the magic part, I'm interested