That's not a direct quote from an FBI spokesperson and instead just part of the subtitle of this article. I personally wouldn't read that it's "private, and not necessarily anonymous" as it seems likely that the author didn't distinguish so strongly between these things.
To the point of anonymous cash transactions, I strongly suspect there are ways to acquire cash in a way that the bills which are used in a given transaction aren't necessarily traceable to the individual who made the transaction. I've heard mention of such tactics but never tried to put them into practice because I don't care enough (in most cases) to make it more than theory.
