Complex sentence: SWR causes feedline losses to be multiplied; low-loss feedline has minimal loss with an ATU, but a "lossy" feedline-antenna combination with an identical SWR may have significant loss.
Split into simpler sentences: SWR causes feedline losses to be multiplied, but with low-loss feedline there is very little loss when using an ATU. However a "lossy" feedline-antenna combination with an identical SWR may have significant loss.