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I read somewhere that in Curse of the Wulfen, it says that if a codex has the same unit(s) as another codex, they may use formations from that codex if they consist if these units (or something similar) (I.E Grey Knights and Librarius Conclave)
Can anyone Confirm this?

There's some poorly worded/missed out bits that don't properly link the description of faction symbols for datasheets in general to apply also to specifically formation datasheets (which is also found elsewhere in other 7th edition codices), but it should be abundantly clear that Formations' units should be drawn from the codex or army list relevant to the formation's faction as identified by the formation datasheet's faction symbol.