[Google Pixel 2 XL Questions & Answers] TheSt33v:
I stand corrected. This is what happens when you flash a factory image:

target reported max download size of 536870912 bytes

sending 'bootloader_a' (36344 KB)...

OKAY [ 0.807s]

writing 'bootloader_a'...

OKAY [ 0.371s]

finished. total time: 1.181s

rebooting into bootloader...

OKAY [ 0.001s]

finished. total time: 0.001s

< waiting for any device >

target reported max download size of 536870912 bytes

sending 'radio_a' (60428 KB)...

OKAY [ 1.344s]

writing 'radio_a'...

OKAY [ 0.578s]

finished. total time: 1.924s

rebooting into bootloader...

OKAY [ 0.001s]

finished. total time: 0.002s

extracting android-info.txt (0 MB) to RAM...

extracting boot.img (40 MB) to disk... took 0.144s

target reported max download size of 536870912 bytes

archive does not contain 'boot.sig'

archive does not contain 'boot_other.img'

extracting dtbo.img (8 MB) to disk... took 0.020s

archive does not contain 'dtbo.sig'

archive does not contain 'dt.img'

archive does not contain 'recovery.img'

extracting system.img (1936 MB) to disk... took 7.975s

archive does not contain 'system.sig'

extracting system_other.img (568 MB) to disk... took 2.816s

archive does not contain 'system.sig'

extracting vbmeta.img (0 MB) to disk... took -0.000s

archive does not contain 'vbmeta.sig'

extracting vendor.img (349 MB) to disk... took 1.626s

archive does not contain 'vendor.sig'

archive does not contain 'vendor_other.img'

--------------------------------------------

Bootloader Version...: TMZ12b

Baseband Version.....: g8998-00164-1710262031

Serial Number........: 710KPXV0398618

--------------------------------------------

checking product...

OKAY [ 0.001s]

checking version-bootloader...

OKAY [ 0.001s]

checking version-baseband...

OKAY [ 0.001s]

sending 'boot_a' (40960 KB)...

OKAY [ 0.909s]

writing 'boot_a'...

OKAY [ 0.374s]

sending 'dtbo_a' (8192 KB)...

OKAY [ 0.182s]

writing 'dtbo_a'...

OKAY [ 0.076s]

erasing 'system_a'...

OKAY [ 0.072s]

sending sparse 'system_a' 1/4 (524284 KB)...

OKAY [ 12.120s]

writing 'system_a' 1/4...

OKAY [ 2.454s]

sending sparse 'system_a' 2/4 (524284 KB)...

OKAY [ 12.086s]

writing 'system_a' 2/4...

OKAY [ 2.423s]

sending sparse 'system_a' 3/4 (524284 KB)...

OKAY [ 12.027s]

writing 'system_a' 3/4...

OKAY [ 2.422s]

sending sparse 'system_a' 4/4 (410176 KB)...

OKAY [ 9.397s]

writing 'system_a' 4/4...

OKAY [ 1.894s]

erasing 'system_b'...

OKAY [ 0.051s]

sending sparse 'system_b' 1/2 (524284 KB)...

OKAY [ 12.007s]

writing 'system_b' 1/2...

OKAY [ 2.452s]

sending sparse 'system_b' 2/2 (57588 KB)...

OKAY [ 1.322s]

writing 'system_b' 2/2...

OKAY [ 0.257s]

sending 'vbmeta_a' (4 KB)...

OKAY [ 0.001s]

writing 'vbmeta_a'...

OKAY [ 0.003s]

erasing 'vendor_a'...

OKAY [ 0.014s]

sending 'vendor_a' (357912 KB)...

OKAY [ 7.952s]

writing 'vendor_a'...

OKAY [ 1.818s]

Setting current slot to 'a'...

OKAY [ 0.008s]

rebooting...

finished. total time: 82.364s

Press any key to exit...

So it looks like the only time it touches the inactive slot is when it flashes system, in which case it flashes something different to the inactive system partition. Weird.