Even though that is the predominant interpretation of "open source" in the tech community, I think one could argue that the term "open source" has no legal meaning (outside of a license agreement, that is). So for the purposes of this argument (code w/o a license), the term is meaningless.
To expand on what you said: the term open source has a specific meaning and it doesn't mean "the source is available". From Wikipedia:
> "Open-source software (OSS) is computer software with its source code made available with a license in which the copyright holder provides the rights to study, change, and distribute the software to anyone and for any purpose."
Once upon a time most of the JDK had the source available - with all rights reserved. It was emphatically not OSS.