You're suggesting that a foreign country supplied the FBI with forged evidence, and the FBI said, "Well, this doesn't match our own investigation, but I guess we'll just assume it's true anyway, now let's announce it to the world on our official letterhead"? Does that sound likely to you?
It's not their job to do the convincing, they leave that to CNN, MSNBC or FOX. They just give some superficial material to get the ball rolling. There was next to no in depth articulation as to how this was definitely Russia. They're still twiddling their thumbs and saying 'we're confident' it was them.