It's the other way round. This is like asking if running RHEL is part of the specification? Obviously not. But RHEL provides an environment that is quite close to the specification.

So is running a busybox part of the specification? Obviously not. But Busybox provides an environment that is close to the speficiation.

   #!/bin/sh
   exec /bin/busybox cmp

  #!/bin/sh
  exec /bin/busybox cmp "$@"

    Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH, ensuring that the returned pathname is an absolute pathname and not a shell built-in.

Plus, you have to know the absolute path of the executable busybox, not something you always know in advance.

But realistically, the only people that need to worry about the actual location of the busybox executable are the people who write the install script - it would take that as an argument or variable and spit out all the little scripts as an automated process. The current installer already has to do most of this work as part of setting up the relevant links anyway.