How can I see those stats?

Yeah that's where I took the stats from. I spot checked them a bit to see if that metric seemed like it was affected by the recent blackout, but it looked like it wasn't (maybe they lag by some number of days). You could take the #users instead which shouldn't be affected by the blackout, but I thought posts better reflected how much activity they were producing (the first couple listed were really low on users but much higher on posts), but really I think for any of the metrics you pick will make the "keeping reddit alive" claim look rather questionable.

Subreddit posts per day is really not a great metric to track engagement. You probably want comments or votes per day.

Isn't the rule of thumb 1/9/99 where 1% post, 9% comment, 99% lurk?

Or something like that.