Since they didn't test it, they are only saying they won't be liable for issues. In short you would be knowledgeable about speaker impedance and ratings to do such a thing. After that, there's not much issue here. But they are telling you that it's not supported. It does not mean it won't work.
I have just purchased a Panasonic 2.1 home cinema blue ray combo. It came without the original speakers and I intended to use a couple of Tannoy Hi Fi speakers that I have. However, the instructions clearly state that failure to use the original speakers could result in damage to the unit and/or poor performance. How true is this?