Astronomers use metal to mean elements other than hydrogen and helium. Metals in the chemists sense of the word aren't in general necessary. A lot of life is pure CHONPS. However, most complex life involves some amount of metals in the chemical sense (most animals require both iron and selenium for example). And planets which are of low metalicity in the astronomical sense will be necessarily be of extremely low metal content in the chemical sense, since in order to get the actual metals other than just lithium and beryllium require extensive synthesis chains before one gets to them.