How isolation is achieved in isolated channel oscilloscopes?
Generally you're going to find a transformer in the signal path. Some methods use an optocoupler plus a transformer to handle DC-high frequency, and some modulate the input (which can then go down to DC), pass it through a transformer then demodulate after isolation. 

The photos you linked appear to show a transformer and optoisolators on the bottom of the board, with a canned cheap DC-DC on the top of the board to supply power. Probably not the best isolation.