Releasing attribution evidence right now will not happen as that would intrude on the response. Instead you will get signatures for specific pieces of malware or generalized yara rules to look for indicators of compromise. The specific indicator that allowed for attribution may never be released as then this particular adversary could work around it.
FireEye is a reputable security organization that is publicly traded. If they were lying and it was discovered as a company their business would go away rapidly and some of their executives might even go to prison. If you have legitimate evidence that casts doubt on their statements please share it. Finding something that significant would shake up the industry.
Attribution is most likely a combination of evidence and forensic data collected both from the initial breach (SolarWinds) and other breached entities. Things like how data was exfiltrated and to where to who purchased and setup the domain names, any other malware that was loaded (was it signed, compiled, contain a certificate, etc).
While Snowden did reveal interesting things, there is zero basis to support anything other than a Russian state actor at this time.