Yes you can't retroactively change licenses, but it's also important to know that just because you can read a source file, the file in question is bona-fide open source or free software.
Many people lack this knowledge from my experience.
Solely being able to view source code without other rights absolutely doesn’t make it open source. In general if something isn’t under an open source license it isn’t.
ADDED Unless it’s public domain and then it effectively is.
We had a Windows CE dev license back in the Cambrian Era which included visibility of the source tree, but God help you if you tried to change it and make your own build.
Many people lack this knowledge from my experience.