It also mentioned a false flag op to make Ukraine look like it's been developing nuclear weapons. News just broke of that today from Russian media, entirely in line with the letter. Huge coincidence if it's fake.
I've only seen this being vouched by one guy on Twitter (Christo Grozev), or maybe I'm missing something? It seems very authentic, just very weird that an FSB guy would publicly post something like this.
It was sent to a contact who then published it on their behalf. They were operating as a whistleblower, as I understand it.
But this piece isn't really long in information density. What mistakes could be made? This piece says very little except asserting that Russia cannot win this war and that their intelligence analysts were duped into preparing scenarios they were told wouldn't be used in practice. This can be told in a single paragraph or stretched with prose and "color", which is what happened here.
The piece is very short in this sense, and has very little content that could be botched by mistake.
This article from almost two weeks ago, noted that while it was at that time new for the claim to be used associated with urgent action, it had been made several times before.
https://www.nytimes.com/2022/02/23/us/politics/putin-ukraine...
The claimed "fact" that bellingcat somehow has FSB contacts that are willing to verify material seems like even bigger stretch.
Grozev was a traditional investigative journalist before he worked for Bellingcat. They have also worked with The Insider and many other traditional investigative journalism outfits in the east. It is not at all implausible that they should be able to contact two FSB people for feedback (especially when one of them is retired).
(Not writing this to suggest anything about its authenticity, just adding information)
In this case it sounds like QAnon, just a bit better.