No, you are absolutely right. Heroku is basically just like a VPS. Only that the compute units are flexible, and scalable, so you can add more compute power when you need it. It’s the same concept as Amazon’s Elastic Compute Cloud, or Google’s AppEngine. In this case, since the tablet is the only client, I can get away with using Heroku’s single, free hobby/developer compute unit.
The web app running on Heroku does not speak to the SmartThings hub. Remember that all the SmartApps execute in the cloud. In fact, the SmartThings hub is simply a gateway between the cloud and your local Z-Wave/Zigbee devices. The web app running on Heroku speaks to that SmartThings cloud.
@Ben just posted a tutorial on how to create a SmartApp, which other services and apps can “talk to”. It’s a really powerful feature of SmartThings.