Earlier quoted context omitted.
That only started to be true in 1988, when the US adopted the Berne Convention with its copyright law. In 1981 or 1983, anything without an explicit copyright declaration was considered public domain.
I'm looking at the Gosmacs source right now, and just above the skull-and-bones warning it says "Copyright (c) 1981,1980 James Gosling" If nothing else, Gosling had a valid copyright. Mind you, there were no license terms on any file and it was distributed as source code.
[1] https://law.freeadvice.com/intellectual_property/copyright_l...