Never heard of Xonotic before, but a quick look at the website, its says: "Features such as... ...a functioning anticheat system", While I cannot vouch for if it works (sounds like it since you have never met a cheater) its better than Minecraft where there is no anticheat and only runs at 20 TPS, which is why the SS tool market even exists in minecraft, Games with a cheater problem or where a bunch of things are handled client side (Like Minecraft) there's gonna be external anticheat solutions. CSGO is another example, since VAC bypasses are widespread solutions like FACEIT and ESEA have been made, these are all external solutions to prevent cheaters. Minecraft is also Java meaning its very easy to decompile yourself, it only has minor obfuscation aswell, so yeah its not source, but its as good as source. (http://www.modcoderpack.com/)