One way to understand it is like a poetry anthology.
Suppose a publisher wishes to publish a book of poems from different authors. In the finished product, the individual poems will remain copyright their original authors. The book as a whole (which poems are selected in which order, the preface cover, and graphic design) will be copyright the publisher. I am pretty sure that is right. Check with a good old-fashioned publishing copyright lawyer ;-)
That is the situation here. Martin is asserting copyright over the aggregate work, but not the individual contributions. Of course, the situation with code is more tangled than with poetry. Very rarely is one poem written by modifying the text of another. New features in software are often written by taking what is there already and modifying it. This is the magic of an open source licence. Each contribution came with a licence saying that anyone could modify it in future providing the modification was also GPL. Therefore, it does not matter if you can disentangle the history and work out which specific contributor owns the copyright on this line (character) of code. You can be confident you are allowed to use and modify it.