The shock wave is what pushed the clouds into the circular formation around the volcano. Hot steam is being pushed out of the volcano, forming the white bubble on top of the ash. You are correct, the white steam bubble is not the shock wave. The shock wave can’t be seen, but its effects are seen by the cloud pattern I mentioned above.
Wouldn’t the heated plume eliminate the clouds in a similar (if not exactly the same) fashion?
Not convinced the white cloud is from steam...possibly a condensation/compression cloud? Looks like a lot of altitude to still maintain steam.