Huh. I can't help with a weight limit.
But I could guess the decal means "no storage underneath." Not likely though.
More likely, I'd suppose, is that the carpeted floor panel in combination with the stainless cover provides extra strength via load distribution, whereas a concentrated load on the stainless alone could cause dimpling. I also wonder about heat, noise, and cargo sliding around.
When I installed my Trunk Extension that was a question that was raised, calling out the reinforced plastic replacing the stainless. The manufacturer of the Trunk Extension, Wokeby, also doesn't state a weight limit. I also made the "sacks of concrete" comparison. So far, nothing close to that, but I've carried tires mounted, batteries, tools, propane tanks, and no problems.
Still, a number would be nice.
2 0 2 1 B M W i 3 1 2 0 A h B E V - O a k l a n d
Wokeby Trunk Extension | Duosida 16A L1 EVSE
SeaSucker Monkey Bar Roof Rack