Obsidian is supposed to be a super tough type of stone (in the Minecraft game, in reality it's like glass, it's brittle and shatters easily, but because of the game everybody thinks Obsidian is tough).
OTP Authenticator implying that it's tough like obsidian makes more sense, than a Markdown Editor.
Of course, this is my opinion and I don't know why either of them called their project Obsidian.