The NYT is a reputable organ, known for checking it's sources. Unlike Wikipedia, newspapers don't generally cite their sources; but if a reliable source states something like that as fact, they've probably checked. You're right: they haven't said they can prove it.

[Of course, it could be propaganda, government or otherwise; or it could be a damned lie concocted by the reporter or editor. They've said they have sources, they haven't named them, so it's not unreasonable to suppose they have sources that don't want to be named.]