How would you classify supply-chain attacks?
Primary security was bypassed by breaking secondary security .. so there was security to be overcome, there was no social engineering aside from understanding procedures in play, and no disgruntled employees.
https://www.techtarget.com/whatis/feature/SolarWinds-hack-ex...
https://forensiccontrol.com/guides/unravelling-the-moveit-ha...
Over time they got more interesting and less like the "basic unsophisticated | opportunistic | social engineer | inside agent" description given above.