I think they are using image segmentation with Deep Learning. The technique should be similar to this. Of course, there will be some traditional CV techniques as well.
I think many mobile device/camera apps have been doing this. The portrait feature where the background bokeh is artificially made is using a similar technique.
Your about page says "sophisticated AI technology to detect foreground layers and separate them from the background" but that's pretty vague.
What kind of AI?