I still don't see it. There's not much in F-SIMs about additional categories and classes. It does say

"For an additional category rating, the applicant must present a logbook or other records showing that the applicant has received instruction in that category of aircraft and has met all of the requirements of § 61.165. A knowledge test is required for an additional category rating."

61.153 says the applicant must hold a commercial cert with an IR, it doesn't say in the same category or class. And 61.167 says the holder of an ATP has the same privileges as commercial with an IR.