Plenty of companies have fucked up things that don't deserve the benefit of the doubt on the next project yet I'm sure it was still given. I'm not sitting here going on hope when I say Halo 5's multiplayer is awesome. That 3 week beta was the best thing they could have done to show us how far the game actually is with gameplay polish and connections.
Ya I should think that when you have a beta like 10 months before launch and it's already working as well as it did, that should eliviate a whole lot of doubt. Also the circumstances of bringing 4 games from various periods of time and platforms together under one joint interface seems to me like quite a bit different a challenge than building one game from the ground up for a specific platform. Also, there is no reason to cast much doubt on them when they say that MCC and Halo 5 had/have different teams working on them. They are a huge studio, easily capable of splitting teams/workloads, and one would imagine that Halo 5 (an entirely new game) isn't something you can just delegate to the side for extended periods of time to work on MCC. Frankly I didn't even need to be told that there are different teams involved there, it's just common sense.