Official announcement of the fundraise posted by other media outlets (and more exact funding numbers of $6.6B at a $157B post-money valuation), although this one conspiciously omits who the investors are.
Is it possible that US or any other government has participated in the round which might be the reason?
I think it's more likely they're taking money from Saudi Arabia or some other unpopular source, and don't want to associate themselves if they can avoid it.