Because it is not a cache. High entropy tables require a lot of vacuuming, which adds overhead to actual database operations. While you can set autovacuum parameters per table, they still are not free .
If you need something like Redis or memcache, use that for anything that is heavy on writes and deletes.
If the suggestion truly is to try squeeze as much utility as possible out of a single RDMBS setup, and if you already need and use an RDBMS, then I would consider it as an option. If not, it's an absolutely idiotic choice. And that's the problem with this article - it really comes of as a suggestion to supplant your Redis setup, a dedicated key-value store, with Postgres, a full-fledged and highly complex relational database.
