Eu acredito que a resposta está em como você define "Unix-like". De acordo com a entrada da Wikipedia para "Unix-like", não parece haver uma definição padrão. 1
Provavelmente o motivo mais óbvio é que o UNIX e o MINIX são antecedentes do Linux, tendo inspirado sua criação. 2A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification.
There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given operating system is "Unix-like".
The term can include free and open-source operating systems inspired by Bell Labs' Unix or designed to emulate its features, commercial and proprietary work-alikes, and even versions based on the licensed UNIX source code (which may be sufficiently "Unix-like" to pass certification and bear the "UNIX" trademark).
Torvalds began the development of the Linux kernel on MINIX and applications written for MINIX were also used on Linux. Later, Linux matured and further Linux kernel development took place on Linux systems.
Linus Torvalds had wanted to call his invention Freax, a portmanteau of "free", "freak", and "x" (as an allusion to Unix).
Se um sistema é monolítico ou microkernel não parece ser considerado ao chamar um sistema operacional "Unix-like". Pelo menos, não quase tão frequentemente quanto se o sistema é compatível com POSIX ou principalmente compatível com POSIX.