I'm not an expert, but from the article:

> To be eligible for a deduction, land needs to meet at least one of four broadly defined “conservation purposes.” These include protecting “relatively natural” habitats; historic sites or buildings; land for public recreation or education; and open space (including farms, ranches and forests).

My guess is that since the golf course was primarily for public recreation (or could be), and was mostly "natural" open space (grass), they argued it qualified. Probably not hard it claim it as historic as well.