You are correct that this is a problem. But it's one of many. Having to read essentially all of the supplementary material to simply understand what is currently happening in Halo 5 is a problem. Granted, Halo 4 also did this, but it was no where near as egregious mostly because, unlike Halo 5, Halo 4's story was actually fulfilling and was genuinely trying to tell a story to give more depth to Chief and Cortana.
Halo 5 doesn't....at all. It instead makes Locke the protagonist of what is clearly Chief's story. If the intention was to make Locke the protagonist, then it should not have been named Halo 5 and should have been it's own well told story to intercut with Chief's story. But nope. This would have been fine if the characters you played, the Osiris Team, were cool, likable and interesting. None of them are (with the exception of Nathan Fillion's character and the fact that I don't know his name says volumes). The only character I remember is Laura Bailey's character but that's because she's Laura Bailey, not because the character was good. It's story also made some statements in other games pointless and what they did with Cortana was.....controversial amongst the fandom.
Halo 5 was also a story where not a lot happened at all except for the Cortana stuff. All that supplementary material that you had to read and listen for what? A story where almost nothing matters besides Cortana and the marketing behind it was a complete and total lie. Halo 5 can best be compared to The Last Jedi: Two parts of their franchises that are only solid if you don't look at them as what they are. If you do, they are extremely divisive and derided moments within their fandoms that have done damage to the story, lore and credibility of their franchise.