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I think they mean the interface between the wear material and the backing.

I'm not entirely sure that it is just glue, but the flip side is that the wear material is, well, wear material that wears away over time, so mechanical attachment needs to not interfere with that.

Plus, if the wear material did come loose from the backing, it would likely make a bit of a racket rattling around (like a brake pad without its anti-rattle clip), but it doesn't really have anywhere to go since it is surrounded by caliper and rotor, and would probably work just fine since the backing is there to spread out the force from the piston evenly along the wear material. That force would still get spread out, the wear material might just shift a few mm.