Indeed it can easily be deleted, that's why I said to put the spam detection before the counter.

I assume most of the visits are from normal users, not spammers, so if a normal user has the cookie for the post set then it means it's a page refresh, so don't increase the counter.

Identifying sophisticated spammers accurately is more complicated though. You can't rely on any client side info (user agent, cookies, browser history, screen resolution, OS, etc) because they can all be modified. You can't rely on IP address either, because there are public hotspots used by genuine users also. I think their spam detector is more complicated than this and they have to use it for HLL also.

So, for the genuine users, a counter increased based on the cookie mechanism would've worked just fine.

> I assume most of the visits are from normal users

That's not a safe assumption. If spam prevention controls aren't working well enough, bots start to outnumber humans very quickly.

I suspect the number of Reddit users who run AdBlockers and/or aggressively block cookies is non-trivial.

Indeed, one of the main reasons for this architecture is to measure the difference between what it measures and what the cookies method gives.