Re: I don't know...
"I'm leaning towards "This is BS", but absent any evidence either way, I just don't know."
Let me help a little.
Let's assume it's true that the files have been decrypted by the Russians, Chinese or whoever (RCow). Presumably this would mean that they'd discovered an intentional or deliberate back door in a supposedly solid cryptographic system. This raises a question: how would the UK or US know?
Possibly RCow took some action that revealed it. But remember that decrypting German cyphers in WWII was so sensitive that it was kept secret for decades afterwards. It was also so sensitive that not all information could be acted on & a disinformation operation was run to provide plausible alternative sources. Would RCow be so incompetent as to let slip, by incautious word or deed, what they'd accomplished. It strains credibility.
Alternatively perhaps the western cryptographers decrypted a message by RCow saying that they'd achieved this. The same reasoning applies. Would they then release this story and reveal what they'd accomplished?
I call BS.