99% it's not a mistake, and MB is talking about ONE nebula, in his statement: in fact he is not only saying "nebula" and not "nebulae or nebulas": he is saying "relates" and not "relate", so there should be TWO ERRORS in a row! Very unlikely.
Since his comment came after the discover of the Barnacles in Merope, we can assume two things:
1) he is still talking about the Pleiades Nebula, saying that it is related to their Origin. Mmmm... I don't think so...
2) he is talking about another ONLY ONE nebula where they can be found, and he cannot hint it, because it's related to their origin. I bet on this one.
I hope it's in Pleades Nebula but it's gonna be interesting if they will be hostile.
I would prefer if it there were FACTIONS of them.
Think Thargoid Powerplay
Also, since we have not heard from them in a long time and found NO evidence of them IN our galaxy except NOW Im starting to wonder if their origin is not about their homeworlds or living space but rather:
ARRIVAL POINT.
In lore they were whichspace mastes and lived in another galaxy and it could be a preparation for colonization by them (or invasion)
But unless they all hide underground and NEVER venture into space since the last war/skirmish we should see a damn population bubble of alien origin at least of our size in the galaxy.
And that Nebula feels a bit too small.
Still, by being whichspace masters they might have invented travel WITHIN planets to one planet to another.
Why venture out into space when you can go to INNER space.