You're quite right, actually. The Chelsea TV commentators were wrong. This is from the CC's official site:
In the event of the scores being level at the end of ninety minutes' play in Rounds One, Two, Three, Four and Five an extra half-hour shall be played. If the tie is still undecided after extra time, the winners shall be determined by the taking of kicks from the penalty mark in accordance with the Laws of Association Football.
In the Semi-Final ties, if the aggregate score is level at the end of the second game an extra half-hour shall be played. If the aggregate scores are still level at the end of extra time the tie shall be decided by goals scored away from home counting twice. If the teams remain equal after this procedure the tie shall be determined by the taking of kicks from the penalty mark in accordance with the Laws of Association Football.
So away goals aggregate only counts at the end of the extra time period.