Given that the desert doesn't offer an attractive set of circumstances for flies to find places to grow I doubt nature would have them waste much time there. I am willing to bet that the scientists in this area haven't done much studies on the desert and have merely extrapolated their work from other areas with an untested assumption that flies would arrive in a short period of time.
After finding a second case where all of the evidence contradicts the fly guys and the same case was in the desert I wonder how accurate their assumption is that the flies arrive right away.
Common sense says nature wouldn't waste the time sending flies into the desert and the two cases with two similar time problems indicates a time delay.
You may be right. BUT: How do you explain the golf course which is heavily watered in order to keep the greens green? Do you think flies would find a place to grow around a golf course?