The architecture is actually somewhat different than that. You don’t have to have a smartthings hub at all . Its real purpose is to establish a local zwave and zigbee network, but smartthings is still primarily a cloud-based system and pretty much everything, including the smartthings hub, runs through the smartthings cloud. Most code, even mode changes, runs there. In the future, most code will run on servers provided by the customers and then communicate to the smartthings Cloud through a REST API. So you still don’t need an ST Hub. (Technically you don’t even need one for zwave and Zigbee devices, you could, for example, use Hubitat or Home Assistant for that and then use the API to communicate with your smartthings account.)
Here’s a recent discussion on the architecture: