Sometimes we aren't the boss and we don't get to see the evidence. That doesn't mean there isn't any.
Can you think of any reason a government engaged in cyberwarfare might want to ensure there was informational asymmetry? I sure can.
OK. Has the government indicated that there is classified evidence?
"We have evidence but it's secret". Hey, we've seen this one before, it's a classic!
At this point, the credibility of the trust-me-bro evidence can fall anywhere on the scale between "Iraqi WMDs" to "Imminent invasion of Ukraine", and there is just no way to know.