Bettors certainly will not know about the fix match before the scandal is finally revealed.
I think that bettor will bet in leagues that have interesting matches to watch and usually leagues with scandals like this are unattractive matches and end up losing interest from bettors, but whatever it is if I have to choose I won't bet on a league that is proven to have a match-fixing.
The fewer people that know this, the easiest it is to profit greatly from it. In leagues without an avenue for players to report extortion and threats anonymously, this is very easy to be done. Just get thugs to blackmail the players, and matches can be easily fixed. Unfortunately, Greek league is like that and team owners are proven to be criminals. So it isn't unlikely that many of their matches are fixed by threatening unwilling players. Although, through the years, they might have found ways to make entire teams do it willingly even.