Responses
It's a common practice in story telling, lots of Western stories do the same thing. Not to mention, some of the lore from the two religions sometimes overlap or contrast one another.
Yeah, it's common in Western history too! Quite a few of the famous stories/elements we associate with the Bible aren't actually from the bible, but were added by medieval (and later or earlier) theologists, and the mixing of Christian and Jewish elements too isn't just a thing of this author.
I really think the author just kinda winged it and stuck various demons from Christian and Jewish demonology in there. Even Lilith is a Jewish demon, sometimes considered Adam's first "wife" who refused to obey him (and a popular character in feminism.)