Yes it is anonymous. But it's the intentional misuse of the sacrament to rein in the confessor. Even if the confessor "knew" before, they probably didn't know for sure of any particular situation or details. Now they would have it from the horse's mouth. So if the confessor said anything in regards to it with the pre- confessed information, the sodomist would always threaten to charge that he broke the seal. "Topic' off limits. It's an insurance policy for the sodomist.
The only reason I am saying this is that I was told it happens. (No I was not told of the specifics of the confession- just the practice)