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Do they add glass fibers, I wonder. That's a way to make plastic stiffer but it's a bit harder to make.

They definitely add something. Noctua has a different texture and grain.

Hopefully not - I’d hate the idea of my fan shedding glass fibers right into the exhaust of my PC and onwards into my office.

Shedding is not a significant concern. You almost certainly use glass fiber reinforced plastics in products like your car and power tools.