The number of implants doesn't need to be that great. If you are already tapped into Supermicro's supply chain and ordering system, then you can already figure out when a bulk of servers passing through manufacturing are going out to a certain customer. Bribe someone or have an agent on the floor slip in the implant during assembly. Maybe the gerber files for the boards already have the necessary pads built-in for debug, maybe someone is swapping in modified gerbers. Maybe the implant is installed totally after hours when the boards are already built or when they are on their way to the final assembly facility.