I highly doubt it would be possible to just "add a chip to a motherboard" if that motherboard was not designed for that chip. Was there a convenient socket ready and waiting for this stealth chip? Sorry Bloomberg, the real world ain't like the movies.
But at the same time, I'm skeptical about security screenings of incoming hardware. If a new server arrived shrink-wrapped in plastic, do you think they completely disassemble it and inspect the layout of every circuit before before putting it into production use? I doubt that. They likely do run a software screening, and maybe that has some diagnostic / signature system to verify its integrity, but to physically inspect the hardware seems like a reach to me.