If a ruck was formed Horne clearly could not play the ball from in front of the last feet: Law 16.5:
http://laws.worldrugby.org/?law=16
However, ultimately whether Kepu was there or not is somewhat academic. If there was no ruck, there had still been a tackle, which would have put Horne in an offside position. Law 15.6(d): At a tackle or near to a tackle, other players who play the ball must do so from
behind the ball and from directly behind the tackled player or the tackler closest to those players’ goal line.
Of all the decisions in the match, this is the one that I most struggle with. Most of the other howlers you could put down to something like understandable human error (the knock on) or an unfortunate direction of travel in the way the game is officiated (the yellow card). This one is inexplicable to me.