Yes I too have never heard of a mass hanging in Van Cortlandt or frankly any location in Amer Rev.
30 hanging in one place would be very a large number. In army terms it would be a platoon size group. For comparison there were only 22 Hessian killed in the Battle of Trenton and 30 Americans in the Battle of Bennington. More like a rumor spread by the Van Cortlandts to keep the cowboy and skinners away or today’s Beware of Dog sign.
In addition the quote “that James Van Cortlandt died in his early 50s, a typical age at that time in history is a generalization. Many early Dutch lived well into their 70s and 80s, same for those of the Amer Revolutionary period. In the same vane that those back then were short. George Washington was 6 ft 2.