I hope this clarifies it somewhat. Please note it was not our intention to be misleading and we are working on making the game as provably fair as possible.
There's easy-enough ways to make the jackpot a lot closer to provably fair, like using a (verifiable) delay function on the player-list. Actually it wouldn't even need to be cheap to verify, as people don't need to do so in real time.
As you said, even with that, you could also use house-players to increase your chance of winning. But at least that's something that would be a bit more obvious (amount of volume spikes before each jackpot, making something someone expected to be +EV to -EV).
Anyway, the real fix is just being honest and direct that the jackpots are not provably fair, and retracting previous claims. As really they have none of the guarantees expected of provably fair. I think it's a big deal, considering it seems like the vast-majority of your users are playing for the jackpot (all the 1.01x bets) so it's very important they are aware it's not provably fair in any sense (i.e. you could trivially win them all, or make a particular user lose them all in a non-detectible way)