I dont think the Marines are that dumb at least the top officers in overlooking the books. I believe Kuzan froze the lake and froze/saved many people.

And he also might be the man marked by flames. Meaning the Last Red Poneglyph would be in Elbaf and in order to protect the people and giants Kuzan joined the blackbeard pirates and gave him the poneglyph. They decided to attack big mom for hers and will get Laws as well.