As far as I can tell, they're not telling anyone to deface sites - they are saying that you should black out your sites, and encourage others to do the same.
Wikipedia was/is considering doing something similar.
I may have misread the statement. I read the statement, addressed in part to "Hacktivists", reading:
> Replace the front page of every website we can with a simple, clear protest page.
Given past operating procedure, that seems like a call to action to deface every site you can get access to. If it's not, then I rescind my original comment.
Wikipedia was/is considering doing something similar.