As that rightly states, D-Notices are requests to not publish. They have no legal standing, though they are frequently obeyed. (One relatively reason example was the D-Notice concerning Prince Harry's military deployments — for the sake of his safety and of those serving alongside him.)
What's really disconcerting is that the other outlets have decided this isn't sufficiently "in the public interest" as to disobey the notice.
Does that have more to do with the G. pushing it as a "unique selling point"? I dont know what the press landscape would look like if the indie/etc. had been given the exclusives, but I suspect it would be the same in reverse.
Journalists are particularly cynical people constantly chasing after anything that will get their by-lines in front of faces.. i dont think we're looking at a paper which has suddenly found its 21st C. political morals, but rather a paper that has suddenly found whistle-blowers queuing up at their door.
To look at those papers you wouldn't suspect something of this magnitude is even going on.