In that case you don’t need adb reboot bootloader.
If the phone is already in fastboot mode and fastboot devices sees the phone you can continue with whatever is the next step.
As you said the bootloader is already unlocked (i.e. you successfully did the fastboot oem get_unlock_data step), I guess the next thing is to temporarily boot recovery, so it would be step 4 of the “Temporarily Booting a custom recovery using fastboot” on https://doc.e.foundation/devices/lake/install