“It took him and Aokiji 10 days to accomplish that. The feat isn’t casual nor impressive when you take in account the duration of time.”
That was never actually stated. The duration of the fight was 10 days but that doesn’t mean it took them 10 days to terraform the island. The effects are seemingly permanent, as the island is still burning/freezing 2 years later. That should override the argument of the feat being unimpressive because it took them some time to do it (Which isn’t confirmed).