Except that the source code does not seem to actually be available. The download page that is linked from the license page does not say anything about source code except how it is licensed. Looking at the source of the page shows a commented out section that talks about how to get the source code and links here. However the link to the source code on that page is dead.

Also, the license has an exception for the GPC code, which is free for non-commercial use only. Admittedly, I don't know how much functionality it enables in Paint.Net.

So I would say an MIT license without any actual source code available is less than free.