Incorrect.
In the 70's OS software was provided FOC when the client purchased the hardware.
That does not mean that it was in the public domain.
Since then MVS 3.8J has been readily available under public domain along with all associated software including compilers etc to anybody who wants it.
Again, making something freely available does not place it in the public domain. That is a common misconception.
However, I did some research, and it appears that some versions of MVS may have fallen into the public domain under current copyright law (which required, at one time, that copyright notices be placed on copyrighted materials published in the past in order to retain copyright protection). Apparently IBM took no steps to ensure copyright protection of some code (in the days when steps were still necessary) and has not attempted to assert copyright in some cases.
The source code for mainframe operating systems historically has been more or less public, to allow customers to modify code and no doubt because proprietary mainframe source code isn't of much use to anyone who doesn't have the corresponding hardware. However, publishing source code is not equivalent to placing something in the public domain.