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 mean

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.