Endgame teams, like one including Lydia and Seer, can definitely run FK25 in a few minutes, so it's just a question of having a lot of gems and letting your autoclicker buy automatic refills... 

Though I believe FK20 is actually more efficient in terms of tournament points per energy, so they might be running that instead. They'd need to do even more super raids then of course, but then it would also take less time per run. 

 It can be whales but it can also be people who just log in and do their daylies but not really work on account development. You can build up tens of thousands of energy quite quickly from your free refills that way but of course that's only helpful if you already have good teams for all the content.

Super crazy fast FK20 teams might be what you're up against. Seer, Coldhearts, Reset Champ like Kymar. Only a handful of seconds to get to FK and then total lockdown and smite of FK.

Keep in mind that when whales have such crazy numbers in champion chase, the majority of the points is coming from their epics. It's not 160 pulled legos, it's more like 20 pulled legos and 280 epics, or something. With them probably just feeding most of those epics right away. It still takes hundreds of shards obviously, so still requires spending many thousands of dollars, but it's not quite as crazy as the numbers you'd get if only calculating the legos.