I think the use of "interface" is sensible, if somewhat ambiguous on the surface. In this context, options & arguments are the interface to the utility, not the "CLI", which is the interface to the command line itself.
Context-aware tab completion is very doable in the unix world, actually; it's just a hassle on the part of the utility-writer. Have you ever noticed that if you add a new file "foo/bar/baz.py" to a mercurial repository and type "hg add fo<TAB>", it will autocomplete all the way to "foo/bar/baz.py", without stopping at each directory? This is because hg has overridden the default tab completion provided by bash and defined its own set of possibilities.
I wish it were easier to set up; I think if that were the case, we'd see a lot more utilities with spot-on tab completion.
Context-aware tab completion is very doable in the unix world, actually; it's just a hassle on the part of the utility-writer. Have you ever noticed that if you add a new file "foo/bar/baz.py" to a mercurial repository and type "hg add fo<TAB>", it will autocomplete all the way to "foo/bar/baz.py", without stopping at each directory? This is because hg has overridden the default tab completion provided by bash and defined its own set of possibilities.
I wish it were easier to set up; I think if that were the case, we'd see a lot more utilities with spot-on tab completion.