When Mayer Bloomberg talked of the ultra-rich moving in, he didn't mean so that the city could literally confiscate their assets, but rather, the city can benefit from the investment that follows. There are great arguments to be had about how much NYC actually benefits from massive infrastructure projects like One57, but the property tax structure is intentional because the city benefits in other ways.
A more interesting analysis would consider the myriad costs and benefits of the construction and upkeep of One57. Obviously there is very significant tax revenue and expenses to the city generated through various channels for a building at that scale. They are not and should not be taxed like single family homes or condos.