Not only did you misread my post, but you misread your too.
I never said they had 5 years to "test" co-op I said they had (over) 5 years to implement it and they did, that's a fact, and they announced it from near the start of the project. In this sentence specifically you say they didn't have 5 years, and then said I acted like the project was finished 5 years ago? What? Are you evening reading what you're writing? No.
I had the same thought when I read your post earlier, you absolutely directly literally said 5 years to test it. I thought that was a strange way of putting it.
Look, 343's failure is genuine, and one could EASILY say 5 years is more than enough time to deliver a complete Halo game that includes split-screen that was conceived and planned correctly from the beginning.
But well, it wasn't. And it's just not accurate to say they had 5 years to test the split screen, or even implement it. Mind you, a bit part of the challenge is graphical and the game got a major facelift after the disastrous reveal. For all we know, split screen was working then and suddenly it was plagued with all sorts of technical issues from the revamp.
That probably isn't how it went down but I'm just trying to get across that co-op isn't exactly something you can just "work on" in parallel with the rest of the game to get it "ready" and then it's "done", you can make one change in the game and it could break the co-op.