...I wouldn't say the lore was set up to have much room for you to make your own backstory.
I mean you can if you /want/, but 4 didn't seem to be any more "solid" in how the lore was than 1-3, it just introduced new elements. Master Chief...has always been quiet and essentially empty as far as a personality goes, 4 is hardly the game to start ragging on him for it. Heck, Chief had more personality in 4 than the others I'd say. As far as I can see, the Forerunners were always going to be hyper advanced aliens, it even felt like that's what everyone thought they were In Universe too. This isn't a universe with Gods and Magic.
I dunno, someone could likely correct me on a few of thse things, I'm limited to the games. But with something like Halo I think basing stuff on just the games is okay, as that is it's primary outlet, and the story presented in them shouldn't rely on stuff thats in the expanded universe without explaining it in the games.
I liked when Master Chief was quiet. I believed it was because he was like me, full of interesting thoughts about life, the universe, and everything, but with the understanding that no one cared. Kinda like Clint Eastwood's characters, they always seem to know what's going on, but have absolutely no intention of bothering anyone with it. In 4, Chief was a stupid child who didn't understand his own limitations.
As for the Forerunners, it's like they said in the article- they were better in my imagination because they never got pinned down to anything real. This trend keeps on happening- the Space Jockeys were explored in Prometheus, the Observers were explained in season 5 of Fringe, the Reapers were justified at the end of Mass Effect 3, and the Force was measured in Phantom Menace. Each time, a strange and slightly terrifying thing beyond our understanding turned out to be incredibly mundane. The Forerunners in Halo 1 were an alien race no one knew about who left behind massive structures that often featured beautiful earthlike landscapes, a parasite capable of toppling civilizations, and a weapon capable of wiping out said parasite along with a big chunk of the galaxy. We didn't know why they did these things, and we didn't know where they went, and we certainly weren't a part of it. In Halo 4, we're shown that the Forerunners almost got beaten by prehistoric humans, who they'd already been living with. Considering how powerful they make out the humans to be in Halo 4, it's like there's nothing left to fight. There's no tension any more, because I already believe that the humans can beat the Forerunners. I went from feeling incredibly small in Halo 1 to feeling like the biggest thing in the universe in Halo 4, and like I said before, I like Master Chief to be as small as possible.