I believe the intended meaning was "a language that nobody else knows" rather than "a language where code is impenetrable to everyone but the author." OCaml is pretty easy, but not if you don't know it.
Also virt-builder is very procedural: https://github.com/libguestfs/libguestfs/blob/master/builder... It's "do this, then run this, and if this condition is true do something else". OCaml's strengths here are its brevity, speed and safety. No complicated features are used, eg. no functors, no difficult use of functions as first-class objects ...
(I'm not actually a fan of functors; I think they obscure the flow of the code, in the same way as using inheritance in OO).
While functors aren't something to us every day, aren't they more of an equivalent to the Decorator pattern in OO (which is more about composition and inheritance of interfaces as opposed to inheritance of concrete implementations which I agree is something to avoid)?
That said, I think I see what you mean -- I've found myself using functors needlessly because I've previously used adapters in their place in Java; instead, it may have been for it for me to ask myself what the real problem I'm trying to solve is.
Huh? FORTH is one of the most simple programming languages around.
We're somewhat limited in terms of dependencies. We can add a dependency, but it needs to be available on all platforms we support (Fedora, Debian, Arch, SuSE, Gentoo, OS X would be the main ones). Of course this is an argument against OCaml, but so far we've been fine needing only a handful of libraries and liberally calling out to C.