Nope, I am using the :sis_user_id in my command, it was finding the correct records. As I said in my initial post, the call was returning a valid admin object paylod for the user with a workflow_status of deleted, but then when listing the admins again the record was still there and active.
As my follow up mentioned, I created a new admin with the same role, used the same code to remove them without issue, it was just the existing records from a process we ran last year it couldn't remove (even though the API returned that it was successful). I had to manually delete the records to resolve the issue. Not sure if they were manually created and there was a sticky effect on them, or the API was just failing to remove the records and reporting it succeeded, but if I was using the wrong API endpoint values like you suggested it would have returned a Not Found error, not the matching person's admin object.
I'm all good after removing the records manually, all new records created are correctly being deleted (no change to the code).
As to my original subject on the post, I manually typed that out, my actual API endpoint has the sis_user_id in it, just forgot to type it.