While it may never be known who the targets were, information revealed by NSA leaker Edward Snowden revealed the existence of government programs designed to do exactly the thing Apple fears.
The National Security Agency's Tailored Operations Access unit was, and may still be, tasked with redirecting shipments of servers and routers headed for targeted organizations to government facilities. The packages would be opened, compromised firmware installed, and then the packages re-sealed and delivered.
One NSA manager described the program as "some of the most productive operations in TAO because they pre-position access points into hard target networks around the world."
Photos which accompanied the leaks showed intelligence agency workers modifying Cisco gear, infuriating the networking giant. Cisco later announced that it would address shipments to empty houses to avoid government tracking.