Do you think understanding the tools, infrastructure, coding styles, activities, targets of these groups allows them to perform attribution?

No. First they ignored all the other intruders on the DNC network because they didn't fit the Russian narrative, then they took a Chinese tool of choice like X-Tunnel and claimed it's some custom Russian tool, then they forgot to tell us that the actual email exfiltration had nothing to do with the internal intruders - it was just phishing, done from the outside.

These private digital forensics companies act more like PR companies, so trusting them with something you can't verify is silly.