Yes, that was my idea as well. They must have some sort of cache system for serving basic user meta-data at scale when a page is loaded, and they could add a time-expiring list post ids of the posts viewed by a user to do detection on a per user basis on the backend.

I think they want to break it into different services for (whatever) reasons.

Running a counter across a sharded in memory cache implementation (like Redis) for something like post views should be ok, but they might have some weird rule-sets for spam detection that are slow.