Supplies from above (Chapter Approved 2018 mission)

Hello everybody,
yesterday I played my first battle with the new CA missions and we had a discussion:
In this mission models with the FLY keyword control objectives even if they have fewer models surrounding it then the opponent has. if the enemy models now have the objective secured (or a derivative there of), do these rules cancel each other out or does the mission rule take precedence?

I queried this at a tournament the other week, before the game started. It was going to be an obvious issue! It was decided that the mission rule overrides objective secured. But that was a decision by the TO and I don't have anything in rules to back it up.

yet most of these objective secured abilities have sentence written in them thats says if enemy models have a similar ability the cancel each other out
weird
i find these new missions somewhat bad written and unbalanced

Played this mission aswell a couple weeks ago; for whatever reason we just assumed it shifted the obsec rule from troops to FLY rather than granting it to both, it would have made it completely irrelevant vs my Tyranid opponent if he kept obsec on his swarms of gaunts and Genestealers. My fluff logic for it would be the objectives are falling from the sky so FLY units can grab it before ground troops.
Was a very fun mission to play since I was trying to stay away from him and cap objectives but as luck would have it they kept floating towards his big monsters, always just out of reach without sacrificing myself!

My club ended up ruling this as Mission trumps, Codex next, then Rulebook.
So in this mission, FLY takes priority, Troops have priority over everything else, and then its normal.
We might be wrong though.