What is Public Domain?

Software that has been put into the public domain is freely distributable. The author has relaxed the copyright to allow anyone to copy and pass on the game or application, but not to incorporate the code into your own programs. At the time (Issue 89 of ZZAP! was released in late 1992), several public domain libraries collected and distributed Commodore 64 software, selling them for a small postage and handling fee.

By law, any creative work you produce is automatically your own copyright, or if you're making it on behalf of a company, it's that company's copyright. The only way a piece of software such as the Stix/Quix game could enter the public domain is if the copyright holder chose to make it so. This had clearly not happened, so the game was still copyrighted. The fact that it was a decade old and long forgotten made no difference at all.