That is what I would mean by “open source” but I can’t blame the uninitiated from thinking it means something else. Consider every-day usage of the word “open” - an open door could be fully open, just have a small gap or even shut but unlocked (“come in, the door is open”). A well-meaning developer could think Unreal engine is open source because they can see the source code (the code is “open” to them). Words don’t have innate definitions, they have usages.
With the minimal amount of work added the combined work can now have added restrictions. They’re pushover licenses.
Devs are free to choose whatever license they want but in the pathfinding problem of interacting with others then “protecting the source” is the wrong target node. Copyleft is a tool to help people.