I know that not to be true. American citizens can enter Canada by land or sea with a photo ID, proof of citizenship that could be a birth certificate or certificate of naturalization, and evidence that you'll be accepted entry into your onward destination. It's the American CBP that require a passport or WHTI-compliant document. If an American citizen showed up at the Canadian border with an un-enhanced drivers license and a birth certificate, they may be turned away because they Americans might not accept them back, not because the Canadians require a passport. Since the GE card is a WHTI document, and thus proves citizenship and eligibility to reenter the US, it stands to reason that it would work, but I can't find a definitive source or a firsthand report.