That's maybe because, since he's also selling commercial license, Qt must be paid for(Qt is only LGPL/GPL for OSS). Some people refer to anything not "completely" free to use, as proprietary.
It funny because his software would be "proprietary" in case that's what he actually meant.
I agree, I think there's some confusion as to what "public domain" means in this context. This project is open source software, but not public domain software.
also for some reason Qt is listed as a "proprietary" dependency in the readme