That might be correct if the statement had said "taken over". But it didn't. It said Cityflyer had been "nibbling away". And that is what has been happening. Even IATA would recognise it was the same passengers on the same city pair.
It's not nibbling away in the sense they're eating into BA's market share, it's BA's conscious decision to move some point to point capacity out of LHR. Remember LCY-GLA wasn't a success when BRT launched it in the first place, it took a strategic decision from BA to push traffic away from LHR towards LCY to make the numbers add up.
If LHR does get a third runway, I suspect that position will change perhaps in the opposite direction.