That just means it was copied without permission... like this code, which is by default All Rights Reserved. For maybe more on-topic examples, though, note companies that are very careful about employees looking at open source code.
While this code is "all rights reserved", you do have the right to view it according to the GitHub terms of service. This is not true for Windows.
Alleging that Wine authors have participated in illegal sharing of source code would be a powerful accusation, and the authors clearly want to avoid it.
