IAAL (but this is not legal advice).

Anyone can freely license a work to the public, and copyright holders were doing that long before modern computers were invented.

“Open source” (other than, say, in the context of open water sources or intelligence or journalistic sources, where it was rarely used) as a descriptive term did not enter the common lexicon until 1998 and that was specifically to refer to software source code.

https://opensource.com/article/18/2/coining-term-open-source...