Pretty sure he's right. I think this change was made during the last legislative session. I think a warden can still check a guide and their clients for fish and game violations, but can't check the outfitter/guide for their guiding license.
I've been checked on multiple rivers, including the blackfoot last summer. They checked for guide license and which outfitter I was working for, proof of insurance, fishing license and state commercial use permit, and that clients had valid licenses to fish. Unless this has changed the last few months I think it's another urban legend.