That's one interpretation, but it's not really what people mean when they refer to "one tool". The progenitor of the expression is Doug McIlroy:

    This is the Unix philosophy: Write programs that do one 
    thing and do it well. Write programs to work together. 
    Write programs to handle text streams, because that is a 
    universal interface.

Interoperability is part of DOTADIW idea, but it flows from the "one thing": If each tool does one thing, you can compose them using pipes. Of course, interoperability also implies tools can be swapped out, though as with DOTADIW, it's more of a myth than reality, because reality isn't that simple.