SO is still good, what they mean is that it has peaked or in the process of peaking. Just like previously-cool bars or festivals are still good but just 'not the same' and on the verge of becoming bad. It's a very specific mindset where you must be growing (not just growing, the growth rate must also be growing) and avant-garde all the time, and it's not really surprising to see it on a forum about startups.

I'll also add that although SO was of course invaluable to me as a reader, the few times I've had to actually ask questions because I couldn't find what I needed, other users were stumped as well. Just adding my little anecdote to yours