You're right, but first the INSTALL script makes a few customizations based on what OS you're running (Solaris, OS X, Linux, *BSD), then bootstraps mk and finally runs mk on all the application source.

The script takes into account differences in operating systems, without including thousands of lines of weird little leftovers from 30 years ago when you needed to check if you were actually running on a 16-bit Data General machine or whatever (I'm looking at you, autoconf)