Hey buddy! Thanks for the kind words 🙏
So long story short, it's been a 2-way-thing:
I worked as a Designer for Microsoft (France) when I was a teenager and I stayed in touch with some people from there over the years. When I started working on this project, I reached out to them and ask them if it was possible to work on this without having to worry about legal issues, and they pointed the Xbox Game Content Usage Rules. We checked and regarding what I was working on, everything seemed alright to keep going since I wasn't breaking any rules.
A year later, when we announced the game and release a first playable version, we learned some people from Halo Studios got their hands on it. Following this, the Director of Communication approached us and got in touch, very nicely, to talk about the game and making sure we could keep working on it and share it without making the Legal Team at Microsoft uneasy. They worked on their side to verify everything and came back to us saying we got their approval to keep working on it and have it playable for everyone everywhere we wanted, as long as we updated some stuff and especially our Terms & Conditions with a sentence/statement they gave us, within the game credits and on everywhere we were publishing it.
So nothing crazy to be honest haha, their approval/blessing is mostly us being safe that the game wouldn't be shutdown and that we could keep working on it without having to worry about legal issues.
Hopefully it answers your question 🙏
PS: Fun fact, one of our prototype was about recreating the CE's campaign as a 2D game in our style, and when I discovered your project, I thought it was better to offer something else to the community so they can get different fan-games to play, so keep up the good work buddy! ❤️