I'd add "it takes money to make money".
Investing based on gut feel
If you can't de-risk such an early stage startup, then we're basically talking about gambling to some extent. Yes social proof works well and gut feel (based on experience) works too, but many of these type of investments won't work out.
I would've done the exact same thing as Yuri Milner, but I don't have that kind of cash laying around somewhere. So that'd be a huge gamble and risk for me. But for someone of Yuri's wealth, this is an incredibly smart strategy.