Its because build 12 didnt have iron. Build 13 is the Cave update with tentacles, the Twins, and has iron in the cave. (At least thats what im believing on because build 12 was supposed to have tentacles and other stuff, but it didnt.
Got confused myself too when i read the development log post of build 12 since it introduced the cave and tentacles, but there was no tentacles in the cave. Turns out that was for the Patreon build 13.
Fortunately its now updated to build 13, but the new development log post is confusing the order of what is available for the public and what is for Patreon.