Plagiarism, by definition, is taking the work of someone else without attribution. If you’ve provided attribution, it cannot ever be plagiarism.
Note that this is not the same as copyright infringement. If I upload the complete 3rd season of Knight Rider to YouTube, that’s copyright infringement, no matter what. But if I were to do it and say “created by Glen Larson for NBC” in the description of every video, it would not also be plagiarism.
The above site cannot be plagiarism because every single one points back to a specific XKCD comic or comics that it used as its source. It could be copyright infringement, although I suspect it would probably qualify for a fair use defence due to being parody.