Sorry for answering this late!
I feel like the Mabinogion has a bigger picture that has a bit of arthuriana in it, like an overlapping, or even more, like a big mix of texts and traditions where then we have one of them (arthurian related one) take a new road (a bit like an evolutionary tree).
About the Chronicles of Prydain (I love those novels), I think because not many people consider Culhwch and Olwen as 100% arthuriana, it would probably not be considered an arthurian novel. The arthurian theme in Culhwch and Olwen was the (very common) theme of having the story happening in arthurian times, with arthurian characters in the background. If I remember correctly, they did take that out from The Chronicles pf Prydain.
I think it mostly depends on how much people consider Culhwch and Olwen to be arthurian characters instead of only overlapping with arthurian characters that then remained in future arthurians texts.
Arthurian Legends 