If not, then
tanru has nothing to do with it. Metaphors don't need to be
tanru and tanru don't need to be metaphors (the conflation
of "tanru" with "metaphor" is another case of Lojbanic mixed-up
terminology).
tanru is conflated with one kind of metaphor, what we eventually called
a "binary metaphor".
But not all tanru are metaphors.
There are things that are both tanru and metaphors.
There are things that are tanru but not metaphors.
There are things that are metaphors but not tanru.
There are things that are neither tanru nor metaphors.
That's similar to "green" and "car":
There are things that are both car and green.
There are things that are car but not green.
There are things that are green but not car.
There are things that are neither car nor green.
Using "green" as a keyword for "car", just because there are some green
cars, would be silly. Using "metaphor" or "binary metaphor" as a keyword
for "tanru", just because some tanru can be metaphoric, is about as silly.