If everyone is on trunk, then a team communicates their changes to each other through the trunk.
If everyone is on a branch, a team is tempted to use their branch to communicate their changes to each other just on the branch. Next thing you know, you've drifted far from main, and another team is trying to touch the same files, and there's no way to merge, and someone who did a refactoring has broken your APIs, and you're tempted to release from your branch because you don't have time to reconcile the merge, and you have code that solves this problem in ANOTHER branch and you manually copy it in to this project in the same place, but it needs to be slightly different so they drift apart from each other, and then your project gets deferred and none of your code gets merged...
If you're on trunk, you HAVE TO commit in order to share your code with your team. And that makes all the difference!
It's possible to use branches the same way he's proposing to use trunk, but it's awful tempting to do bad things.
I'm reminded of the Indian proverb I saw on Reddit: "If you want to go fast, go alone. If you want to go far, go together." Meaning, if you quickly want to make a demo, a branch is your friend. If you want to make sure your code lives on, make sure it lives on in trunk as quickly as you can.