Assuming it was Russia, and that they did derive it from some secret means, public announcing they discovered it but not giving the source makes that inference

Assuming it was Russia, and that they did derive it from some secret means, public announcing they discovered it but not giving the source makes that inference more likely. That's why eg in World War II, the allies only acted on Enigma intercepts if there was a plausible alternate source. So my inference is that they're lying about having sufficient evidence to definitively point at the rooskies, or they're being reckless with sourcing.