From context it is obvious that what was meant is that the English army being involved in supporting, or conducting, a guerrilla war was still a foreign concept in 1806. After all, guerrilla war, although not called that until apparently 1809, goes back to antiquity.

To make it clearer for you: To be fair to Fox, Moore, and Stuart, at the time, the concept of supporting/conducting a guerrilla war was entirely foreign to the English army.

The issue may have been more to do with undisciplined irregulars fighting without centralised command and discipline, often engaging in what they regarded as murder Moore and Stuart were professional soldiers after all.

As we all know, only 2 years later in Spain, this principled reluctance on the part of the English army to support a murderous guerrilla war was inoperative. I presume that it can be agreed that Wellington was also a professional soldier.