But I can't think what else could be involved other than thermal damage. Thinking about it, I also wonder how much thermal stress is required to produce damage. If only a small increment in thermal energy at a micro-level or at particular chemical bonds is what happens, it would be a challenge to detect and measure such tiny, localized changes. Then again if the effect (that is, RF agitating some chemical entity) is small enough maybe calling it "thermal" wouldn't be exactly right.
Will be very interesting to see how this line of research develops. In any case, a great question that requires a lot more work to decipher.