SunOS, HP-UX, and BSDs up to a point all ran System V binaries until recently, or still do. If they really want that then it's a thing they can ask for.
Speaking as someone who used to work in dev at Facebook, in my time there I saw absolutely no Unix systems, just Linux on the production side. They did have some proprietary systems in corporate IT that could have been running on HP-UX or AIX for all I know, but a PE intern wouldn't ever have touched those.