So, the round result is based on the current block, and if I'm not wrong that information is accessible to anyone, I'm not an expert on the topic, but you should have a server seed generated by the site on a random way, and not based on public information.
How would the public know the RNG is not rigged if that was the case tho? If I understand it correctly, Keccak is quite popular for RNG in Solidity[1]. It is not like a user can predict the block difficulty and timestamp as they wish, and even if they somehow know it, there is still RNG at play. At the very least, knowing these three factors doesn't necessarily mean you can predict which numbers will be chosen by the smart contract. At least that's how I understand it from the code you shared, do correct me if I'm wrong.
[1]
https://www.geeksforgeeks.org/random-number-generator-in-solidity-using-keccak256/