i only look at referrersMappings, not at mixedReferrersMappings to throw the exception. the Generic document maps the mixed referrers. as the mixed referrers are immutable anyways, it does not matter. and for something like the Generic document it really makes sense to do it, we don't know if its referenceable or not.
for the editable referrers it makes less sense to map on a document that is not sure to be referenceable - it would still work if the document is made referenceable on PHPCR level.