"... one major union actually played it at their conference.."

Actually, no. Many major unions played it either at their conferences or at the discos after the business of the day. Whatever the original intentions of the writers, those unions at the time adopted it wholeheartedly and sang it lustily, everybody having both arms raised in celebration and solidarity. I know, cos I was there. Great.

Certainly beat the execrable "tie a yellar ribbon round the old oak tree".