So not sure if this has been addressed, as there are a crap ton of articles and replies to go through, but I got an idea from reading a response in one post. Two things could be true the vault isn't finished and just like the dlc there are placeholders already setup or these things have nothing to do with the raid and will be part of some new quest still serving as placeholders.
To me the latter makes more sense than some undiscovered secret. Most likely the gate outside at the beginning of the raid will connect to the one by the platforms. It will be accessible to people outside of the raid and it will be a separate instance for the group accessing it. The quests discussed in the dlc announcement will probably utilize these areas as one part of the mission, much like a part to our exotic bounties.
I'm not saying this is a definite thing obviously, but for me it seems like the most likely reason for these things to exist in the raid already. If they have placeholders for strikes and other areas then what would be so surprising about them doing the same here? Anyway that's my theory, hope it makes sense.
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Lol just no.