Can these experts make these claims with 100% certainty? Why or why not?
You can almost never prove anything 100%, but for all intents and purposes you can be quite certain. Can we know for sure the Shadow Brokers leak was legit? No, but we can conclude any other option would be highly impractical.
But the American government is not providing that level of proof for the situation you mention. I have yet to see a shred of technical evidence of this particular statement.
Say, the american government suspects someone of hacking a presidential candidate. They could probably easily compromise their node with tools from the intelligence community. This would provide proof, and it would make all the bellicose rhetoric sound a lot less insane.
As far as i can tell, the most solid reasoning in this case is:
“We believe, based on the scope and sensitivity of these efforts, that only Russia’s senior-most officials could have authorized these activities,”