Yes, the novella and the game books are very much offical but they don't fit into the timeline that was later established. They probably never meant to anyway. Like Koutei said, there was no person in charge back then, so they could do whatever they wanted. I suspect these works are included as alternative timeline stories, similar to how Circle of the Moon and Apocalypse games are still included.
Although Nightmare of Blood has little to do with the series, I think the two game books fit in way better and have very fun and original storylines. I especially like the Vampire Hunters because I think it captures the atmosphere from Castlevania 3 very well, arguably being a better sequel than Curse of Darkness was in that respect.