isnt isn't available on NT, so a script or person can tell which platform they're on by trying to run this command. isnt with the -doesnt flag simulates a program which isn't available in a non-NT environment.
The -doesnt flag doesn't reverse the logic when invoked on NT, however, since isnt doesn't exist on NT. This is a feature rather than a bug because the intent of the -doesnt flag is not to reverse the logic of the program but to neutralize it. In other words, isnt without -doesnt will tell you if you're not running on NT. But using isnt without -doesnt will allow you to operate in a portable way that doesn't force you to distinguish whether you're not on NT or not.