They are real, but not all of them.
The NetEnt ones and Microgaming (MegaJackpots, MEgaMoolah) are 100% genuine. They drop often to the largest volume areas of players (Sweden) for those games. And often Micro go in Asia due to their market presence.
HOWEVER – RTG slots (RealTimeGaming) and those casinos are dodgy. Their Jackpots regularly drop to people on max cashout bonuses (I won one myself) and to those on free spin deposit offers. There was proof of this 6 years ago but they may have changed now, still… I would never play there.
Jackpots drop less likely than you win the lottery, unless you play high high stakes, then its just slightly less than winning lottery 🙂