Is there any reason why Nocturnal Emissions / Nyctophobia is listed as a compilation as opposed to an album made up of two recording sessions (as is the case with Black Desolate Winter / Depressive Hibernation)? The additional notes suggest that each part was released separately on CDr, but I am unable to find any evidence of this and the official Striborg book makes no mention of it.

Dunno. But does the official Striborg book mention details about other CDr demos? Or does it list extensive discography? Or are there any contrary claims?

There is a section for every release and mentions every different version. Only the Finsternis and Displeased versions of Nocturnal Emissions / Nyctophobia are mentioned. I suppose it's possible that some separate CDr copies were made, but it looks like someone just inferred that based on the way they're presented on the CD.