Well, this is the situation as I know it;
Retro (Not Nintendo, remember, Nintendo would be making Metroid Dread, and Retro made Prime) would have put it in there for various reasons:
- A complete coincidence, i.e. Retro didn't know about the "Dread" rumour.
- An inside joke, to tease the Metroid fans, and stir up the fan communities (IMO the most likely)
- Retro really do know that Dread is in production, and it is really nearly done (IMO the least likely)
Seeing as I seemingly remember Retro "debunking" this or something... I don't think we'll see Dread, and I don't think it's nearly done. Although, the Prime series is finished, and unless Nintendo want to kill the Metroid series (again) then the next step would likely be a new 2D game, namely Metroid 5.
Matt Casamassina, when asked about Metroid Dread in a podcast on the August 24, 2007, mentioned that "a wink to the fans" about the game was in Metroid Prime 3: Corruption, although he clarified that this was by no means a confirmation of the title. This was later explained as an in-game message in Metroid Prime 3: Corruption (by scanning a panel within the Pirate Homeworld's "Metroid Processing" room) stating "Experiment status report update: Metroid project 'Dread' is nearing the final stages of completion."[45]. In an interview, Pacini stated that this was a coincidence and not related to the rumoured game[46]. On September 6, 2007 Nintendo said they are "not making the 2D Metroid at this point in time." [47] This does not, however, rule out another 3D Metroid game.
Linky