Two things. One: "play to the whistle" isn't an absolute. There have been plenty of instances where a player has taken the initiative to play the ball out because an opposition player seems to have taken a serious knock. It's not just about heads.
Two: in this instance the Leeds bellend deliberately slowed the play down making it look as though he was going to play it out, before flicking it down the line. He not only fooled the Villa players, he fooled the linesman as well, who lost track of the offside position. On this ground alone the goal was a cheat, and Bielsa was right to give us a goal.