You can play around with the validator at:
http://validator.idpf.org/
I tried substituting various things and it seems like the long list of Sigil is wrong, at least according to the 2.0 standard.
I don't think that Flight Crew has ever been a high priority for the Sigil developers. It is good for catching big errors, but it doesn't check to make sure references exist or are correct etc. It is convenient for that, but I always try to use epubcheck just before posting an epub.