I'm pretty sure that the mystery slot on the back of the Xbox Series X is a CFexpress memory card slot. If an ethernet port is about 11.5mm wide then using the
Xbox Series X: Mysteriöser Anschluss soll SSD-Speicher erweitern - WinFuture.de
Modder uses CFexpress adapter to attach Xbox Series NVMe SSD as expandable storage - VideoCardz.com