I just skimmed a mueller report. All instances where mentions of the hacking group names/numbers were, contained just assertions of what the group did. There was no to very little mention of how this information was determined in the surrounding text.

Yes, I guess it's a report. But it contains just assertions, so it's only meaningful if you trust the institution making it. I've read reports that actually contained the process the core conclusion were arrived at, like various opensource investigations from Bellingcat, etc. And these were indeed more convincing.

Not sure why hacker should just go by blind trust to institutions, lol.