Whatever Cohen was peddling Mueller wasn’t buying. How can you use the testimony of a man who has no concept of what the truth is to begin with? Who is a known liar? Any evidence Cohen could give would be tainted.
You cannot use the testimony of a known liar in a courtroom where there will be cross examination, but you can surely use his words in a REPORT which is what a special prosecutor will write. This entire witch hunt is Stalinist.