I read the docs and I think I understand how the immune system works, but I think it is still a real problem. If I have enough malicious nodes then it is possible that I will be all of the randomly selected validators. I send some coins to A, and when I spend this money again to B (double spending), all of the validators will say "it's correct". It's possible because I can start an unlimited number of nodes "freely". Btw, this attack could be prevented if validation would be incentivized, and validator nodes have to stake some coins (something like PoS). In this case, running 1000s of malicious nodes would be expensive and very dangerous.