I have opened the following issue in epubcheck explaining the situation. Hopefully they will modify a future version of epubcheck to do "the right thing" whatever that is.
My hope is the powers that be will formally adopt the draft "backlink" and epubcheck will support it properly or maybe return "referrer" to the standard vocabulary list.
One or the other should work.
https://github.com/w3c/epubcheck/issues/1617