@kinyutaka Hey, thanks for sharing, it looks indeed free but not published under a license that will perpetually make the software free (and I can't test, no Linux release).
The Free Software I love (eg. Krita/Kdenlive) is "Free/Libre and Open Source" (but English often simplifies it to "Free"). They are published with a license that constrains all contributors to open the code and make it available, all the time. It gives to users a lot of freedom and benefits.