Ha, I would mess up the example. :P You're completely right of course. I guess I assumed it applied to both genders, but I'm still picking up the gender of a lot of the words; that's not terribly intuitive.
Almost all the other nouns are masculine - el carro, el niño, el oso, el barrio, el avión, el camión, el árbol.
Pretty sure with those 2 simple rules you will catch 90% of genders right.
Manus, hand, is one of an even smaller set of feminine fourth declension nouns. I believe this oddity has also been preserved in Spanish as "la mano".
This makes me wonder whether the Spanish words for trees tend to be feminine while having masculine-looking forms; that is regular in Latin.