Chimney sweep here, you got it 100% correct, thanks for raising awareness about this! We install counterflashing on houses that don't have it now and then, and we like to use sheet copper for it because it just comes out looking beautiful. Another thing we see very often is that when roofers install a new roof, they will tweak on the counterflashing and bend it out really hard to be able to get the step flashing under it easier, and it's usually okay functionally, but it leaves the counterflashing very bowed out, and sometimes it can damage the metal, break sealants, etc. I wish all the inspectors that referred me knew as much as you. It'd make our jobs much easier!