Winco, I wonder if it's simply a case of being ultra-careful?
If the team getting 558 airborne start advertising the fact that they're about to conduct the first flight, it is inevitable that people will turn up in some numbers to watch.
And it would seem - sadly - that it'd be just as inevitable in the current climate (where you can find someone mean of spirit close at hand wherever you turn) that somebody would file a complaint that the publicity was a means of circumventing the spirit of the ANO - i.e. although not billed as a display, that's exactly what the first flight was. Otherwise, the argument would be, why publicise it unless you wanted the public to turn up to watch?
It might also be contended that by publicising it, the event was being made open to the public, even if they were having to jam up the lanes around the airfield to get a vantage point. While I would personally view such a complaint as petty and - to borrow from Sir Humphrey - 'placing a greater epistemological burden upon the English language than it can reasonably be exepcted to bear', I can equally see why the team might not wish to run the risk.
Also, if - as has happened on several occasions in aviation history - some snag materialises on the day of the publicised first flight and the Vulcan can't get airborne, imagine how disastrous the
PR would be.