Hmm. I interpreted "[not publishing source code] goes also against the GPLv3 license that you're using" as meaning that the author is "bound" by the license to

Hmm. I interpreted "[not publishing source code] goes also against the GPLv3 license that you're using" as meaning that the author is "bound" by the license to provide the source code. Of course, the terms in GPLv3 that puts limits on the licensee / user (like that derivate works must be GPLv3) are not relevant. But section 6 in the license stating that the source code must be distributed still holds true. Either OP distributes the source code to people who have the binaries, or the work is not licensed under GPLv3?