Complex sentence: Using low-loss feedline with an ATU leads to very little loss, despite multiple reflections, but a "lossy" feedline-antenna combination with an identical high SWR may loose a considerable fraction of the transmitter's power output.
Split into simpler sentences: Using low-loss feedline with an ATU leads to very little loss, despite multiple reflections. However, if the feedline-antenna combination is "lossy", then an identical high SWR may loose a considerable fraction of the transmitter's power output.