The back is likely liquid metal if this is real, because it is the only rational explanation for there being a seamless metal casing. No, they wouldn't put antenna in the Apple logo, they haven't thus far, and they wouldn't start now.
Here is some old info on liquid metal soon after Apple licensed it I believe.
"First order of business is an iPhone case that incorporates a new antenna. The alloy case could be structural as well as functional. In other words, a Liquidmetal case could also function as a big antenna. And because different mixtures of metal can produce alloys with different characteristics, Apple could blend an alloy that is optimized for receiving radio signals.
You can build casings with functional characteristics, and the alloys properties as an antenna can be optimized, Peker said.
Apple could also make cases with very complex and unusual shapes, Peker added. The cases would be literally seamless."
Link: http://www.cultofmac.com/54827/liquidmetal-inventor-exotic-alloy-may-make-new-iphone-antenna-exclusive/
If this is liquid metal and they are going to use sapphire, with the addition of the extremely reduced bezel, and the what appears to be substantial thinning of the device, it is extremely innovative and amazing.