The limitations of this approach are worth noting. First, if the intruders never activated their backdoors, then there would be no evidence of communications with C2 servers. Hardware inspection would be the main way to deal with this problem. Second, the intruders may leverage popular Internet services for their C2. Historical examples include command and control via Twitter, domain fronting via Google or other Web sites, and other covert channels. Depending on the nature of the communication, it would be difficult, though not impossible, to deal with this situation, mainly through careful analysis. Third, traditional network-centric monitoring would be challenging if the intruders employed an out-of-band C2 channel, such as a cellular or radio network. This has been seen in the wild but does not appear to be the case in this incident. Technical countermeasures, whereby rooms are swept for unauthorized signals, would have to be employed. Fourth, it’s possible, albeit unlikely, that NSM sensors tasked with watching for suspicious and malicious activity are themselves hosted on compromised hardware, making their reporting also untrustworthy.
The remedy for the last instance is easier than that for the previous three. Proper architecture and deployment can radically improve the trust one can place in NSM sensors. First, the sensors should not be able to connect to arbitrary systems on the Internet. The most security conscious administrators apply patches and modifications using direct access to trusted local sources, and do not allow access for any reason other than data retrieval and system maintenance. In other words, no one browses Web sites or checks their email from NSM sensors! Second, this moratorium on arbitrary connections should be enforced by firewalls outside the NSM sensors, and any connection attempts that violate the firewall policy should generate a high-priority alert. It is again theoretically possible for an extremely advanced intruder to circumvent these controls, but this approach increases the likelihood of an adversary tripping a wire at some point, revealing his or her presence.
— Read on taosecurity.blogspot.com/2018/10/network-security-monitoring-vs-supply.html
An assessment of the Bloomberg hardware compromise report which provides insights I hinted at but are better articulated here.
I remain skeptical this happened. It seems cheaper and easier to introduce fear, uncertainty, and doubt (FUD) into the supply chain than to actually compromise it (beyond what the Chinese supply chain already does to skim money). Again, time will tell.