It sounds like the surface team didn't dedicate time/priority to adding an NFC antenna and instead focused on the case. The quote is below so judge for yourself, but i don't read of an actual problem with the Mg case and NFC, I read it as - the case was critical in our dev process, nfc was not.
For the product design experience we were aiming for with Surface, the Mg metal enclosure, including the back case, was critical. This made good antenna design for NFC a trade-off in our development process.