I may be misunderstanding something, but the non-replica rank 14 items are showing up in ATT as collectibles, such as the items from the vendor Lieutenant Jackspring. Since purchasing these items do not unlock the transmogs, it seems these should not qualify as "collectable."

Collectible, but can't be used as transmog.

Still counts.

What makes it collectible in a way that's different from every grey and white-named item in the game?

Grey and White items do not return a "collected" == true state from the API.

Okay, so there's a hidden flag that gets toggled. Makes sense now.