This is not speculation or propaganda, though. The information security community knows, not  everything, but a lot about the computer network exploitation capabilities of most large nation states. China, like the US, runs this as a military capability, but their teams are a lot more decentralized than American hackers. Russia is quite different altogether. We know the general methodologies that each prefers (which often reflect the organizational structure), the kinds of tools each prefers, and we know the types of targets they go after.

People who study this have access not only to forensic analysis of breaches, but also get a lot of data from accidental leaks, and often the attackers' servers are compromised, with contents then publicized. US and Cdn intel services know even more than is public, because they directly attack Russian and Chinese hacker groups.

You can choose not to believe it, but that is not something you can reconcile with objective facts.