They have nouns and verbs. I'm not well versed in the emoji kung-fu (I'm too old), but I've seen strings on them on Facebook that have meaning. Like a cow followed by a turd. A birthday cake for happy birthday. That sort of thing. The character set is less than 1000 - and as you point out, has a totally different origin. When it hits 8,000 it will be up to a literate Chinese level, 20,000 or so to meet parity.
(I'm not suggesting emoji will ever become a full-blown language. I'm just pointing out that an ad-hoc collection of little glyphs with no alphabet can in fact become one. Dismissing glyphs as "stupid little graphics" is arbitrary since they are clearly filling a communications need.)