Aye, my assumption has always been a flowery reference to a place/time when all feel treated well where they live. (Maybe lots of milk & honey helps too.)
The literal 'Promised Land' was given by God to Abraham, therefore the Jews have the greatest historical and spiritual right to be there, it is now ironic that the Muslim world, which was only created around a thousand (?) years later, arrogantly believes it has some divine right to 'wipe them from the face of the earth'.
I think the 'Promised land' Martin Luther King was praying for would be when the black people became emancipated.