"Reporting anonymously" is not the same as "The FBI doesn't know who you are when reporting". If reporting to the FBI was anonymous, they would have no way to counteract spamming or flooding of their system. When you report anonymously, all you're telling them is "please don't contact me about this".

I don't know how to provide a source for something as basic as this. That's like asking to provide a source that air is air.