You have problem to understand easy things like this.
They have 3 ways to push the birdcage without being damage in Dressrosa
Coat CoA on a weapon
Seastone
Bari Bari no Mi
On this panel.
Marines goes with zoro's Team. This team doesn't have seastone or Bari Bari no mi.
So they used CoA on a weapon.
Anime also support that claim.
I mean marines don't have others choice since they dont have bari bari no mi or seastone.
We also know marine can use CoA.
Now you can deny it or accept manga facts.