Derrin is slightly confused by this, but manages a plausible decoding: Dath ilani may individually have such means or motive, but their entire civilization has never before been faced with the temptation of a whole other Earth, full of people that the smartest dath ilani (if not average dath ilani) could perhaps outwit, and without the power of organized resistance.
"Would you, Isabella, take all that we have in dath ilan, if the power to do that lay within your capability alone, and we could not oppose you? And if not, why not?"