Immutability, the way I think the author means it, doesn't mean data can't be changed ever, in any way. It just means that modifications(e.g. corrections) cannot completely overwrite previous values without leaving any trace, but instead have to appear as amendments, visible alongside the previous versions of the data.

If it was entered wrong, you can issue a correction, but a trace of the first, wrong value will stay available.

That means nobody can just say something was never what it was, just that it changed(and then have to justify why).