Is it really free?

Yes:

There are no license fees
There are no hidden premium features you have to pay to get access to
There is no pay wall
There is no requirement to share your email address in order to download
There are in practice no restrictions on neither output nor bundled graphical assets (all core asset such as the base mesh, targets and skins are shared under CC0)

The add-on as such is open source and can be downloaded from GitHub.