The fact that we saw that he made that comment just shows that he knows exactly what he's doing.
He is the head honcho, nothing gets broadcast without his say-so, so he knew that letting us know he'd said that would only add to the publicity. It's not like the person who typed up that subtitle so we could see it did it off his or her own bat; they were told to do it. They don't type up every comment made, after all, for our inspection and consternation
If Simon Cowell hadn't wanted us to know that, he wouldn't have given it the go ahead. And if he'd really not wanted us to see him saying it - for fear of vigilant lipreaders - we'd not even have seen him making a comment to whoever sits next to him - Amanda, is it? Might have been the other one, Carmen, I suppose.
Another thing to think about is that he might not even have said it when that kid came on, he might have said it about someone else but then got it edited in to where he knew it would cause maximum impact. He might have said it about no one, solely to get it edited in for broadcast, wherever he saw fit.
Telly like BGT is all smoke and mirrors and Simon Cowell is the master of audience manipulation.