Anyone who has worked on planet scale anti-abuse systems knows this is a very tough and never ending problem.
You think there are obvious signals for good or bad - the 0.01% cases where this does not hold up turns to 20 million daily mistakes if your product has 2B DAUs. You think you built something that works, spammers need to figure out just one loophole to game the system. Sometimes they don't even have to look at a technical loophole, if the economics works out you can even pay normal users small sums of money and the bad activity is now masked by troves of genuine user activity.
It is a little painful to see how HN loves to vilify people working on these issues, but its a little like saying we have police but the crime is not 0 yet. And if the argument is it happens too often that is actually most likely not true - a 99.999% accurate system will make a mistake 600k times every month for 2B DAU activity assuming one user interaction a day for the product. Which is often a big underestimation for many large products.
I do think like any other area there is a lot we can do better and that we are already working on it. But man it sucks to have spent 6 months working 12 hour days just to see someone making grandiose statements on how something is an easy issue to fix when you know its absolutely false.
I guess its fine though, FANG pays well and I enjoy my work and think its a net positive to the society.
/rant