Its not a real dimmer! 
Its a rotary mqtt controller.
As you quite correctly say a real dimmer needs zero crossing detection, FET/triac etc.
This is a smart bulb controller that looks and acts like a dimmer, but really is sending mqtt commands to HA, which in turn controls a smart bulb like Hue. In the video the bulb is a hue bulb and its 100% NOT being conventionally dimmed by the sonoff - it being dimmed by HA controlling the Hue bridge based on MQTT commands from the Sonoff.
If you know of similar that can also dim a 240v incandescent or LED load I want to know as I have some rooms with expensive 240v LED downlighters that I don’t want to replace!