Not going to click the daily heil link, but what a silly headline.
In any family where dad went to school A (all boys school) and mum went to school B (co-ed) and the kids are a mix of boys & girls the parents are going to seriously consider school B as a sensible choice that they can send all their kids to as and when the time comes.
When the family in question have extra issues to deal with around security and publicity a campus-style boarding school is a better daily environment than a day school.
Marlborough seems like a perfectly rational choice to me and describing it as a "snub" to eton is just stupid journalism because the word is nice and short and the actual meaning of words unimportant to the rag in question.