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.
IANAL, but I think open source started with software since software has source and binary form. Now with compression and other shenanigans, probably even videos or images could be argued to have a source and binary form. I don't know a thing about multimedia, but people here saying this "open source" release is a good thing mention specifically the fact that it's the uncompressed version, or as the FSF would call it, "the preferred form of the work for making modifications to it".
You’re correct but words and phrases can evolve in their meaning over time. If the licensing terms for this are analogous to open source software licensing terms then calling it “open source media” is pretty reasonable.
It's awful what happened to literally. The enormity of the change in meaning is so egregious. When it literally gets used with both meanings in the same conversation, decimating my brain, I have to wonder how nonplussed anyone trying to learn English must be. I'm sure there are plenty of words it's happened to, but this must the most egregious example.
