according to a rescent report, the story states that it was the Kokura area, the zone targeted for bombing, but that the bombers had to attack the secondary target after steel mill workers burned tar to create a "smoke curtain" over the city. Source: http://mainichi.jp/english/english/features/news/20140726p2a00m0na014000c.html
I'm not sure how this is a 'new allegation', the document setting out the locations considered in the official bombing order of July 25, 1945 was declassified in 1974.