There is notice on the website not to use version 1.0, and to check your version using specified command.

However, the command is incorrect.
It says

pngquant -v

when in fact there should be capital V

pngquant -V

The version of posterizer provided in ImageAlpha is different from posterize as it exists in this repo. This becomes evident when running the following:

$ posterize -v 64 test.png test.png
MSE=20.288 (Q=43, 64 levels)
$ wc -c test.png
210715 test.png

$ /Applications/ImageAlpha.app/Contents/MacOS/posterizer -v 64 test.png test.png 
MSE=17.192 (Q=50, 64 levels)
$ wc -c