You're only comparing the Amazon instance cost with Heroku's full bundled cost. The article is including provisioned storage, IOPs, backups and data transfer - all factors in AWS pricing - to the included levels with that Heroku tier. AWS comes in around $1,156/month on demand when equivalently compared to everything in the $1,200 Heroku plan.
If AWS pricing was that simple, the comparison would be a whole lot more straightforward.
Heroku:
Amazon In simpler terms, Heroku is more than double the price.