Harry's patronus, a flint?

[theadimail]

No: HPFGUIDX 93508

Hi,
  In OOTP, in the Hog's Head chapter,Fred and George say that they 
did not tell others about Harry's patronus-bringing ability, because 
Molly had warned them not to.  And it is implied that the others 
didn't know anything about this fact.
  But, in the thired book, Harry invoked his patronus in front of a 
stadium full of quidditch watchers to ward off Malfoy and gang 
posturing as dementors, remember? How could it not have gotten out 
that Harry knew how to invoke his patronus after that?
Bye
ADI