Once the diffuser is off, you'll find a PCB, held by two Phillips head screws (guess someone has stashed a bunch of screws for themselves, hehe).

Don't make a rookie mistake. Always use the correct screwdriver bit (PH0), and hold it tight all the way out. To prevent stripping the screw head, apply moderate pressure down the screwdriver with your palm, twisting it with your fingers

The screws are out. Here comes the know-how. My guesstimate was incorrect. The PCB cannot indeed be pulled out from the case without further prying. However, there is an easy way to pry the case open cleanly. Put a knife inside the gap between the PCB and the wall. Pressing against the PCB, put the knife at an angle, and push the wall outward.

The goal is to detach the wall from the base. You should not need much force to hear it unglue with some cracking noise. Be careful with the knife. Use eye-protection. Once you repeat the procedure with the other sides, the base should come loose. Watch the hinge. Once the base is loose, the whole package can be pushed downward.