A little digging around suggests 35kg per square meter is a more reasonable estimate[1], which is about 8lbs per sqft. I suppose it's possible some measures include the weight of the water in the mussels, and others don't?
That's ignoring the buoyancy from displacing water and only covers two years. I could see the worst case say a stationary house boat being vastly more problematic over 30+ years. But, they should generally fall off before the weight gets crazy.
1: https://www.boatdesign.net/threads/boat-weight-increase-due-...