Chthonians are part of the Lovecraft mythos. Think landsquids. Cthonic deities are a type of Earthly deity in Greek mythology, contrasted with the Heavenly Olympians. A decent enough analogy might be the two types of Norse divine beings: the aesir and the vanir. Recall that creation was divided three ways when Saturn was slain: Neptune got the ocean, Pluto got the underworld, and Jupiter got the Sky, after a fashion. In this sense, the Cthonic Gods were Shadow-Gods or even Bizarro Gods.
I am certain the copyright on Lovecraft has expired, and the copyright on Greek paganism doubly so.
Drow, however, are content covered by the
Open Game License. In other words, Drow are part of the SRD. They have not been exclusively reserved by WoTC like other D&D elements (e.g., the mind flayer, if memory serves.) However, WoTC is rather fickle with their Open Game Content, and some manner of nod to D&D would have to be given. This nod may not be something Dulrik wanted to give.