But the article is 3 days old. It seems many did not expect that much unity from subreddits going dark. 2.5 billion affected subscribers is quite something!
I'm still in hopes they change their mind in light of recent events. Don't think they will though.
Probably 95% of that is overlapping since one person being subbed to 20 participating subs is counted 20 times, and since some default and massive subs are participating there must be a ton of overlap.