That sonoff device communicates using Zigbee, not MQTT. Detach mode is a custom Zigbee attribute configurable via a custom cluster (apparently it’s cluster 0xfc11 and the boolean attribute 0x0017, although there’s one more attribute, a bitmap 0x0019, related to that feature so maybe there’s more to it).
An Edge driver would need explicit code to read/write that attribute and expose it to SmartThings, maybe there’s a custom driver around or one that you could ask the developer to modify in exchange for a coffee.
Maybe you can use Mariano’s new tool to configure that attribute: (EDGE Driver-Mc). New Zigbee Device Config Mc driver