I think you're misreading skepticism. Most of the world lacks any background or education to be able to look at and analyze the attack, and will accept whateve

I think you're misreading skepticism. Most of the world lacks any background or education to be able to look at and analyze the attack, and will accept whatever their told about the source and nature. People that run in more technical or security-related fields might be privy to more tools in the analytical toolbox. It's probably also quite likely that trust in US defense intel is not at an all-time high, exacerbating said skepticism.