Signature plastics had issues with PBT scratching on the dark gray color when trying to produce the "grayed" SA set. It's a thing I guess
Historically PBT is beige and used with dye-sub, so dark PBT keycaps are less common. Could the dark pigment be responsible for weakening the plastic? I've never heard this being a problem for Topre, but then I've never seen this issue mentioned before in relation to any manufacturer. The idea that the nanobiosis coating is stronger than PBT is curious though.
This is the sort of information that everyone should be adding to the Deskthority wiki with references, but instead, it appears and then disappears again, becoming impossible to distinguish from hearsay, slander, misinformation and plain old Chinese whispers.