No not like Destiny, he said what he said like Halo CE. Anyone that knows/Played CE knows what he is talking about.
When you land on Halo you have to rendezvous with the crashed lifepods and save marines towards the end of the mission there are 3 bases where the survivors are you can save any base in any order you want it's open but the mission story is still linear.
You can even stealth and avoid most of the first half of the missions enemies.
On the Mission The Silent Cartographer you have the entire island as a sandbox as soon as you touchdown with pelican you can storm the beach or you could turn around and walk the other way and fight the enemies guarding the control room.
Even Halo 2 to an extent with the mission outskirts you could jump to the rooftops of buildings and traverse your way around it may not have been intended but due to the sandbox nature of the mission many gamers did this unintentionally, you could even Grenade jump or coop jump onto the pelican when it came after the hunters and make it to the rooftops that way.
I don't think he saying the missions are going to be open world but sandbox like how CE and 2 were you could traverse certain missions more than one way and it not just forcing you "Go here" "doors closes/building collapses...can't back track" "fight enemies" "door opens/event triggered...you can move pass this stage.
The more newer Halo games came out the more they made the missions linear in approach. So no it's nothing like Destiny Halo has been done this before and fans love the sandbox nature with a linear path to the story.
+ the sandbox nature of some missions is what made skull collecting and terminal hunting so fun. The more linear the missions are just makes finding collectables a chore and not very fun or challenging.