The thing is Halo 5's story will stand on its own, just like Halo 4's did. You understood the aspects of the story fully that the game felt you needed to. If you wanted a deeper understanding of it all, then you would read into the extended fiction.
The same exact thing happened with Halo CE. You learned significantly more about key characters and details about the universe from reading Halo: Fall of Reach. Same happened with Halo 2 with the book Halo: First Strike. It has always been more rewarding to read into the extended fiction, dating as far back as the original xbox. This isn't something that 343i just now started doing imo. What I think 343i is doing a lot better is more strongly tying together information from the books with what's in the games. It still ends up amounting to the same thing more or less. You are rewarded for following the extended fiction.
It wasn't important for people who played Halo 4 to know who Jul M'dama was. He wasn't a major concern then. They will clearly do more to inform players of who he was in Halo 5, as evidenced by Palmer directly name dropping him in the opening cutscene. Everything we needed to know in Halo 4 in order to follow the story, we were pretty much given. If you wanted to dig deeper and have greater understanding of it all, then that's where the books come in. I mean, they shouldn't have to literally spell out everything for us. The games shouldn't have to work that way because the books don't literally spell out every detail of what transpires in the games either. If you want to know more about a particular story thread, then you should seek out the piece of media by which you can acquire that additional knowledge.
I had a similar reaction myself.