Because he attacked the metal wall itself. He should have attacked the foundation of the wall, where the stonework that the wall is attached too meets the ocean floor. If you destroy the foundation, the metal wall would just drop into the ocean.
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So you're saying he should have attacked the ground under the walls? Wouldn't that have just destroyed all the ice just the same as Akainu's attack did?
Wasn't that point after the ice was already melted anyway?
And yeah, after watching the Yibis version, it's much clearer that Aiokiji wasn't saying no to Whitebeard using Haki.