Exactly what I was thinking about. According to Apple: "The sensor uses advanced capacitive touch to take a high-resolution image from small sections of your fingerprint from the subepidermal layers of your skin.". Subepidermal means the layer below the outer layer of the skin. This seems to be a marketing lie/gimmick, as a rubber representation (or wood glue in this case) of a fingerprint seems to work just fine, and that representation is of the epidermis.
in all reality, there's probably truth in that statement though i guess i can see how it would/could be misinterpreted
it means that while the phone is creating/memorizing your fingerprint id, it's scanning beyond simple pressure of your ridges (as in, that's probably what they experimented with first in order to electronically store your print but couldn't get it detailed/3D enough or reliable enough so they had to try other means to get a deeper read)
what it doesn't say is that it uses sub-epidermal layers as a means of identification -or- it doesn't scan inside your body (and definitely not your blood as 'subepidermal' might make some people correlate with)to see if you are you.. it's only used to 'take a hi-res image' but there's no mention of it subsequently using sub-epidermal scans during the unlocking of the phone phase..
of course, this is all open to other interpretations and feel free to tear that
# apart if you wish or see a flaw in my logic..
add-- even if it does use sub-epidermal scanning during the ID phase, it's still not saying anything different than shining a flashlight through your hand.. and yes, that same type of scanning/reading would happen with latex or glue etc.. maybe someone should try to duplicate the hack except use a completely opaque material for the fake print.** (which would make a sub epidermal scan impossible (unless using smthng like ultrasound or MRI tech etc.. but I assume the scan is more standard light/laser based?))
..I guess my point is this.. the sub-epidermal scan thing is (probably) way more to do with reliability/repeatabilty ("oh! hey! it works every time!") as opposed to anything to do with stronger security.. if they simply relied on pressure of ridges, I assume it would be a lot more finicky and definitely not "it just works"
**
add2-- you could probably experiment with the sub-epidermal thing a lot more easily than needing to go through the entire outlined hack process.. just push your finger into some wax to make a mold then fill it with something (glue etc..) seems like most people should be able to do this very easily (i don't have a 5s so i can't try it).. if you can get a working print that way, then try filling the mold with a material that light can't go through (no ideas off the top of my head other than ,say, tar but i wouldn't recommend touching your phone with it).. if a transparent material works and an opaque one doesn't then i think that would somewhat outline what sub-epidermal scanning is doing..