For example, some slot machines are programmed for low winnings and it's not about the RTP. It's just that the mathematical model of individual slots was initially built very poorly.
🏆 For example, my favorite slots are from the following software - Push Gaming, Relax Gaming, Netent.
So you're don't trust with the RTP rate, but you choose those three providers are your favorite slots? that's not make sense, it's just your belief and there's no proof if the three providers above are better than the others. You're just lucky when you played those slots and unlucky on the others.
The providers can know the RTP rates of each slot by wagering more than 1 Million bets, so it's really high.
Maybe he is a whale who bets millions also, but most likely he is not and it is just his impression and inductive reasoning. But there is nothing bad with it, as he's asking for other's opinion to contrast his belief.
In the long run, all games (and slots are not any different) are designed in favor of the house, but since small samples exaggerate randomness one may get the impression that odds in some sites are better than others. Very long time ago I played certain online slots which seemed to be bugged because the win/loss rate was really high to me: most probably it was only my case.