You apparently haven't read the ruling.
1) No, not just Mar-A-Lago. There are several properties where this took place. The 3x the size thing is for one of his properties in NY.
2) For Mar-A-Lago, he bought the property and signed a "Deed of Conservation and Preservation Easement". This basically was him agreeing that the property would only ever be used as a social club. He couldn't divide the property, he couldn't add residences, etc. "In exchange for the easement, Mar-A-Lago was taxed at a significantly lower rate (the club rate) than it otherwise would have been (the private home rate). As such, it is zoned as a social club and not as a residence. So, it doesn't matter that the properties surrounding him with giant mansions on them are selling for 500 million. That property is zoned in a way so that its value is significantly less than its neighbors.
He agreed to his property being zoned this way so that his taxes would remain low.
He can petition the city to change this agreement, but if they say no, then there's not much he can do about it.
That does not give him the right to proclaim that the property is worth 10x more and file financial documents proclaiming that the property is valued at that amount until the city agrees to void the Deed of Conservation.