It's just a retcon to cleanup the confusion for having two gauntlets, no need to overthink about it. From CinemaBlend:
That came about because in Thor 1, the reason that it's in this movie is because someone who went through that movie, frame by frame, looking for Easter eggs was like 'Oh! The Infinity Gauntlet!' And then from that point on, they were like, 'That damn Infinity Gauntlet, what are we going to do with that thing?' It's like, there, but it also [exists] somewhere else in the universe. How do we deal with that? And then we realized like, Odin... Hela goes through and she goes, 'Fake. All of that stuff that's in here is fake.' Basically the thing we were trying to go for was that Odin somewhere along the line realized that everyone knew that the Infinity Gauntlet was on Asgard and in the safe keeping of Odin, then no one would come looking for it. So he made a fake one, and let everyone believe... Basically, a fake launch code.
We brought that over -- like here's an opportunity to take something off the plate of Markus and McFeely, who are doing Avengers 3 and 4, and have plenty else to worry about with I don't know -- what -- 60-plus characters in one movie?
In-Universe, Hela knows a lot about Odin's secrets which Thor and others didn't have any idea about. So we have to assume that she knows this secret too.