The announcement says the fares to Heathrow will count towards the appropriate cap (£12.50). There will not be a higher cap set for Heathrow, even though the individual fares will be higher.
As was pointed out on London Reconnections: "Travel by Heathrow Express and you pay £25 just to Paddington. Travel on Crossrail (Lizzie/whatever) and you can get all your travel for the whole day for half that amount."