on some sense, you're right. but it could be investing in other things (that aren't restricted). They can also buy commodities priced in dollars. or just convert it back. even if they want to keep their own currency low and avoid the conversion, they can buy gold or literally anything
Discussion
True. And they have been. Over the past decade, they have been met sellers rather than net buyers.
But currently that relatively 'safw' 4% on billions is a great return. And leverage.