WiFi is not an IoT standard just a networking one. Smartthings can communicate at a network layer with your sonoffs just fine, but sonoff does not implement any standard for other systems to integrate with their WiFi devices, they are proprietary. They expect you to use the eWeLink app. It is however possible to replace the firmware with one that can be supported in smartthings, there is a very large open source community working with sonoff devices. Or just use the zigbee devices, which -is- an IoT standard.