Oh I see, ... they are declaring the opf prefix on the metadata tag itself. Normally the meta tags do not need an opf prefix when it is defined in the package tag as the default namespace (no prefix required).
Just removing that opf prefix namespace from the metadata tag itself should do the trick.
Sigil's default package tag should look like:
Code:
<package version="3.0" unique-identifier="BookId" xmlns="http://www.idpf.org/2007/opf">
Notice the opf namespace is the default namespace which does not use the opf: prefix.