You don’t follow the license that it was distributed under.
Commonly, if you use open source code in your project and that code is under a license that requires your project to be open source if you do that, but then you keep yours closed source.
He took GPLv3 code, which is a copyleft license that requires you share your source code and license your project under the same terms as the code you used. You also can't distribute your project as a binary-only or proprietary software. When pressed, they only released the code for their front end, remaining in violation of GPLv3.
Although I personally am not a fan of licences this strict, MIT+Apache2.0 seems good enough for me. Of course, that might change with time and precedents like this 😅
And as I said there, it is utterly hypocritical for him to sell snake oil to artists, allegedly to help them fight copyright violations, while committing actual copyright violations.