Ez a gól miért nem érvényes?

You can't walk the ball off of the corner-position, hence the referee's decision to call the foul. However, if you watch it again you can clearly see that Rooney technically gives it a valid 'kick' at the very start- before casually walking away.

As of that moment the ball is in play and can be handled regularly, hence Giggs run was perfectly legal.

So the trick was Rooney sneakily taking the corner only for Giggs to take into a better position. Sadly Rooney did so well he fooled the Ref.