Glad that you have satisfied yourself with the fact that IBM have software in the PD.
I had already mentioned the possibility of software falling into the public domain, but I'd be surprised if IBM ever explicitly released it to the public domain. In the early days of computing, at least some types of software weren't considered very important in terms of intellectual property. The objective of vendors was to sell hardware, and an operating system was just a necessary evil in order to get the hardware sales. Later, operating systems became important pieces of intellectual property in their own right. The potential for sales-damaging misuse of proprietary mainframe operating systems has always been self-limiting, anyway.
IBM do not make the source code of their OS's for example z/OS available to anybody.
For one thing much of it was written in PL/S which is itself only available to staff within Big Blue.
Note the word "historically" in my post. MVS was written in assembler, as I recall, although I never did any tweaking of the OS myself.