I think a lot of AHJ could have some fun with, "...prevent the inadvertant interconnection...in any operation of the transfer equipment."
One could take the position that "any operation" could include using two keys simultaneously. And that's the crux of the whole argument. A real transfer switch can't be put into two different positions at the same time.
Sort of like NEMA reversable motor controllers. Not only is there an electrical interlock to prevent both contactors being picked up at the same time, there is a mechanical bar that will not let one side pull in if the other side is somehow jammed in.
While keylocks are familiar to many of us, and certainly the AHJ, they may seem foreign to some homeowners. Someone's wife, who called the neighbor in the middle of the night, may decide that in order to turn that second lock, she needs to go get the key from the safe. No problem, she trots upstairs with a torch, gets the second key and hands it over to the 'helpful neighbor'. Who promptly 'interconnects' the normal and alternative sources of supply. Inadvertantly.
"Qualified" personnel understand that the key-lock is meant to ensure only one lock can be operated at a time. But someone 'unqualified' may just assume the other key is kept in a 'safe place' and just needs to go retrieve it. OOPS.
A simple slide-bar or other mechanical interlock is more 'foolproof' then keylocks. Keylocks are more for when the two switches/breakers are too far apart for a simple mechanical interlock.
daestrom