the testimony was raised by rollbit, not the player who managed to get the win, and it seems that the players who use big money to play so that they can achieve big wins, not many will show their victory because it could be risky for him and usually the one who shows it off is the player who wants get followers.
But if I'm not mistaken, has anyone ever won bigger than that on rollbit
Some players want to share their big wins while some want to remain silent but as you are aware about the username only your position is not at risk completely and you can do it.But still it's our own preferences and casino needs it as advertising promotion to attract new players.If talking about big wins then yes there must be similar big wins before also on Rollbit like the NFT lootboxes giving away million dollars BAYC and punks.
I think hiding your winnings is quite natural. Especially if the win is very big. After all, there will always be envious and ill-wishers. And yet a person feels much more pleasure from win when other people know about it and they are happy for the winner.