I'm not surprised
In star formation, supergiant stars like Rigel and Deneb are rare but massive, then as you go down the Hertzsprung-Russell diagram (W to O to B to A to F to G to K to M) the star types become more common as mass/size/brightness decreases. Blue B-type stars like Rigel are less common than G-stars like the Sun; stars like the Sun are less common than M-type red dwarfs like Wolf 359 or Proxima Centauri.
The numbers increase exponentially, too: for every Rigel there's a thousand Suns, but a million Proxima Centauris.
So it follows that if smaller = a LOT more common, then it's logical that free-formed planet-sized bodies should outnumber stars, even red dwarfs, by thousands to one.
By this hypothesis, many, if not most of these planets are quite likely to have formed independently of any star system, rather than having been expelled from one in the early stages of it's formation. This is borne out by the sheer number of them alluded to in the article; if they all came from star systems, there would be thousands of planets per star, which is not feasible.
Without nearby stars to blow off their atmospheres with stellar wind during their formation, these planets are also likely to have immense, nebulous atmospheres tens or even hundreds of thousands of kilometres deep, such that their outer surfaces may be far less distinguishable than even Jupiter or Saturn's. A photo of such a world would be an intriguing sight indeed.