[DISCLAIMER, skip this if you’re just interested in the topic. I just wanted to say that I’m aware that analyzing continuity in a game franchise with talking snowmen and people like Face McShooty’s understandably considered pointless by many, but I love doing so, and I’m pretty sure I’m not the only one, so here we go.]
I didn’t think there were any reasons to doubt the canon status of the Holodome DLC, but by now I’m mostly skeptical about its existence. I don’t have much proof against it, but I think the little that I gathered is pretty convincing.
First off, no one ever directly refers to the Holodome: Pre-Sequel NPCs (like Jack), the playable characters, even Athena who’s telling the story, nobody (except for H010-TP, obviously) mentions it.
Then I also stumbled upon these tweets from the ECHOCASTS account, which, even though they should always be taken with a grain of salt, are supposed to be canon:
So, what do you think? Personally, my opinion is that there is no actual Holodome on Elpis’ surface (which is quite a bummer, since it has some sweet architecture, neat enemies and amazing gear).