I'm not a lawyer but it is my understanding of US law that something like what you see in the video does not meet the legal definition of a threat. There is no indication that the offender knows the real identity or location of the person they are speaking to, both of which are required to establish the intent necessary to define something as a threat in the legal sense of the word. Furthermore, the person speaking appears to be from another country, likely the UK or Australia, both of which have different laws than the US. Is Riot supposed to evaluate this situation based on the laws of the country in which they have their corporate HQ, the country the speaker resides in, or the country in which the listener resides? I don't think a lawyer in any of those three countries would advise this streamer to press charges based on the content of this video alone which would indicate that this kind of behavior is not illegal. Perhaps it should be, but that's another matter entirely.

To reiterate, none of this is meant to be interpreted as a defense of what that guy said. It's just to illustrate the point that moderation is not a simple thing to enforce even in situations where a surface level evaluation seems like it should be. It's much simpler to mute this guy or leave the lobby or whatever else you feel like you need to do to protect yourself. The unfortunate reality remains that people like this will always be around no matter what system is in place to minimize their impact. That's not to say that no steps should be taken with that goal in mind, just that when all is said and done you will always bear some responsibility in protecting yourself from content or behavior you don't want to be exposed to.