Cannot flash factory images to Google Pixel 2 XL
Ultimately, I solved this by using a Macbook Pro with a USB-C to USB-C cable.

Previously, I had tried it with two different USB-A to micro-USB cables, using two different micro-USB to USB-C adapters, on a Linux laptop with two different USB-A ports and the then-latest version of fastboot/adb downloaded. Unfortunately, I can't be certain what the issue was, but since it happened consistently with multiple cables (and those cables still appear to be working), my guess is that there was a bug with the Linux version of adb/fastboot.