These breaks seem to have contact with the metal case. Even if made from Liquid metal, they still could not be used as antennae. To me it looks like those breaks are separating those small rounded rectangular parts of the case from the rest to be used as antennae. Makes more sense to me, as a similar idea has been used before in the iPhone 4 (although I still think the iPhone 4 design is the best thus far).
Apple has patents involving fusing metal with other alloys that have different characteristics such as being radio-transparent and possibly non-conductive. Materials can involve Liquid Metal, Sapphire and other weird ceramics/metal alloys.
While Apple has a lot of patents on stuff that they may never use in a product, they sure spent a lot of energy experimenting with those materials in the last few years.
The antenna breaks on this shell seems to be perfectly fused with the rest of the casing, I don't think plastic could've provided such a seamless surface. It may not be pure Liquid Metal, but it may well be some new kind of material.