The article exaggerates... there aren't 20K bodies, but "up to" 20K [1], so just a "vanity number" -- and I can't find any reference for what a likely actual number might be.

Maybe "Megie was tasked with executing prisoners" but there is only a record of a single hanging, ever. [2]

And while the "Hangman's Elm" is cool in being the oldest known tree in NYC, the idea that it was used for hangings appears to be a complete later fiction. [3]

So take your sensationalistic history with a grain of salt, folks...

[1] https://web.archive.org/web/20070703234838/http://www.nycgov...

[2] https://en.wikipedia.org/wiki/Washington_Square_Park#Early_u...

[3] https://en.wikipedia.org/wiki/Hangman%27s_Elm

This kind of hyper-local history is the kind of thing I miss most about living in a city like New York. Here's a similar resource to OP with block by block, building by building history in Manhattan http://www.nysonglines.com/