That is flashing TWRP (adb sideload twrp-pixel2-installer-taimen-3.3.0-0.zip)
First time I have to boot to TWRP (fastboot boot twrp-3.3.0-0-taimen.img) and then flash it from TWRP (adb sideload twrp-pixel2-installer-taimen-3.3.0-0.zip) but I did it with sildeload avoiding copying (push).
A/B devices have to boot to TWRP first and then flash. That part is also wrong in “how to flash” procedure for taimen. Flashing recovery from fastboot is for older devices I think. That is how I flash TWRP on my OnePlus X.