I don't think they have. They have said that when Mata shot, the players were in an offside position and were at that point interfering with the ability of an opponent to play the ball. If Mata had not shot there would have been no offside; if the players had made it back past the second last defender before the shot there would have been no offside.I asked "Why would you allow players in an offside position to obstruct the GK's view when that would be an offence if they're still there when the kick is taken?" and it looks like PGMOL have essentially said it should be treated as an offence.
It is not an offence to be in an offside position and this interpretation doesn't change that. At the absolute most it says if a player is in an offside position when the ball is played and if that player interferes with an opponent's ability to play the ball then some consideration should be given as to whether the player was in that position for a tactical reason.