No, they didn't -- getting the absorption spectrum of a transiting exoplanet is an order of magnitude or two beyond the capabilities of present-day instruments.

No, they didn't -- getting the absorption spectrum of a transiting exoplanet is an order of magnitude or two beyond the capabilities of present-day instruments. They just measured the density. From the density, though, you can get a pretty good idea of what it looks like. There are three basic ingredients which go into making planets: rock (including pure metals), "ice" (a catch-all term which includes all phases of water along with methane and ammonia) and hydrogen/helium mixture. Everything else is too rare to worry about. Knowing (basically) the density of each of these components, plus knowing the fact that H/He mixture won't accrete to a body until its mass goes beyond a certain point, you can put pretty good constraints on what the composition of a planet of a certain mass and radius should be. Of course we could be wrong -- it could equally well be a ball of pure argon or something; however it's hard to imagine how any such thing would form.