The article said they could rebuild the count if they needed to. So something in regards to each click is being stored. If you were using postgres you'd just setup a trigger on that table to increment the click values (stored in another table) as appropriate. No aggregate function needed.
Certainly you can implement this same pattern without Redis. Triggers in Postgres would be a reasonable way to do it. I didn't say you can't do this in Postgres, I said you can't do it with COUNT().
It does indeed sound like they're storing every click: that's precisely why using aggregate functions would be expensive.
