It's simple. An AC signal (assuming 0V DC bias) rapidly switches between both positive voltage and negative voltage (that's what the "A" in "AC" means, alternating). If attaching a DC supply backwards (so it's a negative supply) would fry your device, then feeding it an AC voltage of the same magnitude would also almost certainly fry it. If, on the other hand, attaching the DC supply backwards would be fine, then attaching an AC supply would likely also be fine. It all depends on whether or not the circuit can handle a negative voltage without malfunction/failure. There's also the issue of peak voltage. You said an AC supply with the same RMS voltage, well an AC supply with the same RMS voltage as DC would have peaks that are 41% higher. So again, can the device handle an input 41% higher than normal? If so, then the peaks from the AC signal would be fine, if not, they may break it.