Stage 1 cleared! Onward to stage 2: the sand test of DOOM.

We sprinkle a pinch of sand over the keyboard, type on the keys for a minute, and ...

We don't even have to lift the keycaps off to realize that something is wrong. A few keys have seized up!

Prying the keycaps off, we find that grains of sand have invaded through the corner perforations in the membrane and have jammed the butterfly mechanism.

Conclusion: the silicone membrane adds a significant degree of ingress resistance, but falls short of being fully bulletproof dustproof.