I would also wager that Valve was worried about Microsoft attempting to use “creative” methods to compete with Steam and chipping away at them, like hidden API. Its not like Valve knew that Microsoft’s attempt would continue to flop so hard for decades that they couldn’t even try that.

You do have a fair point on problem of association. I don't think diagnosed mental condition have significant prediction power either. Instead, I believe that occasional mental healthcare counselling on people who we find to be normal will be the one that improve this situation.

Well, to be fair, what they meant to say is "There is no way to prevent this while not changing gun law, state of mental healthcare, income inequality in economic classes, or criminal law and policing issues." which is correct statement overall.