An interesting question which led me to do quite some testing to try and understand how things work in iMovie 10.1.
The answer to your question is yes.
iMovie 10.1 has radically changed how libraries are arranged. In previous versions of iMovie 10, original media could only reside in an event. Any clips dragged and dropped directly into a project timeline would automatically also appear in the event which contained the project. If the project was deleted all the original media would remain in the event. (This is still how FCP X works)
In iMovie 10.1 projects no longer reside in events. If one drags and drops clips into a project they will be stored in that project's original media. If the project is deleted, so will this media be. To keep media available to projects but outside of them you have to add them to an event.
The 'Project media' item in the Libraries pane now shows all media used by the currently loaded project which includes media from events and media directly inserted into the project. It also includes media used from other libraries (iTunes, Photos etc).
I have not found a way within iMovie to move or copy clips that have been dropped directly into a project into an event. It appears that one has to re-import the media into an event. If the original media camera files are no longer available the only recourse would seem to be to open the library (right-click . show package contents), open the project folder then the 'original media folder and re-import or drag & drop from there.
Geoff.