The workout doesn’t specify how hard the overall 8-minute block is supposed to be. I think it would be reasonably to say that the “normal” gear is roughly tempo and the “hard” gear is above-threshold.
If you want to maintain a constant cadence in ERG but change the force (hard vs. normal gear), that means you necessarily need to be changing the power.
So we’re talking 3 x 8-minute intervals of 30 seconds hard, 60 seconds less-hard where, in ERG mode, you try to hold a constant cadence.
That sounds like basically the structure of a criss-cross or over-under workout.