There clearly exists a mismatch between the documentation and actual practice. I too was a CC and a COR with different units for many years and have also been able to assign all “3” key 3 delegates in each unit. This was a couple of years back, but I’m quite sure of this.
More recently (this year, 2021) I’m currently registered as an Asst. Council Commissioner and also a key 3 delegate for our council. When we received a new Council Commissioner, it took some time for his approvals and his registration to completely process. During this interim period I (as a Key 3 delegate) was able to assign this new person (registered as a Council Committee member) to be a key 3 delegate as well. This allowed him system access to the MST features.
After his registration as a Council Commissioner was complete, and officially a member of the key 3, I removed him from the key 3 delegate position.
I believe that because of the upcoming changes to OSM (and the new position manager) There are some updates to restrictions going on in the background as we speak. One possible example of this is the newer, recently added “COR delegate” position, which may have revealed some of the unintended “back door” access (user features) that clearly exist in the system.
I’m for taking a wait-and-see attitude until the new position manager is complete and properly debugged. After that, maybe the documentation will be closer to actual use.
By the way, all of these changes going on lately are not that easy to implement (speaking from experience) I applaud the work of everyone on the team.