I highly suspect that he’s using the Post Formats functionality from WordPress core to do some of this using a custom theme. Sadly it’s generally fallen out of fashion and one doesn’t see it very often any more. I suspect that it’s because WordPress didn’t take the functionality to its logical conclusion in the same way that the Post Kinds Plugin does.
I think some of my first experience with its resurgence was as helpful UI I saw suggested by Tantek Çelik on the Read page of the IndieWeb wiki. I’ve been doing it a lot myself, primarily for posts that I syndicate out to micro.blog, where it’s become a discovery function using so-called tagmoji (see books, for example), or Twitter (reads, bookmarks, watches, listens, likes). In those places, they particularly allow me to add a lot more semantic meaning to short notes/microblog posts than others do.
I do wish that having emoji for read posts was more common in Twitter to indicate that people actually bothered to read those articles they’re sharing to Twitter, the extra context would be incredibly useful. I generally suspect that article links people are sharing have more of a bookmark sentiment based on their click-bait headlines. Perhaps this is why I like Reading.am so much for finding content — it’s material people have actually bothered to read before they shared it out. Twitter adding some additional semantic tidbits like these would make it much more valuable in my mind.
It doesn’t appear that Om has taken this functionality that far himself though (at least on Twitter). Perhaps if WordPress made it easier to syndicate out content to Twitter with this sort of data attached it would help things take off?