It was Deidara who did all the work in taking the 3-tails down. Madara was playing the Tobi role. There would be no reason for Deidara to deny it if Madara/Tobi had actually beat it himself.
But anyway, since it was only a cover picture, I'm gonna take it for what its worth....Just a cover picture with no real significance to the storyline. Especially since the Bijuu are already captured and their hosts (if they had one) already killed except for 8 and 9.