Every valid AMP page includes a <link rel=canonical href="..."> to the canonical URL for the document. If the aggregator (facebook in this case) parsed and linked to the canonical as the publisher recommends via this annotation, you would get the version the publisher preferred. This is how browser extensions that rewrite to the non-amp version work, they extract this URL.

The AMP viewer iframe share button (and share intents) all share this canonical URL, not the AMP url. Google's implementation is trying it's best to get you to that version as well when sharing links.

Link Rel Canonical is an old (2012) standard: https://tools.ietf.org/html/rfc6596