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
ADB doesn’t work in Fastboot Mode, only the fastboot command does.
Every time a guide wants you to execute an adb command, the guide assumes that Android is booted (with enabled debugging, ADB = Android Debug Bridge), or a recovery with ADB support.
Edit:adb sideload is an exception to this, and adb devices works with the phone recovery in ADB Sideload mode, too.
TWRP is a recovery with limited ADB support.
But the command in the guide is adb sideload, which is special in the sense that almost every recovery can do that much (I edited my earlier post accordingly), it just differs how to enable the ADB Sideload mode. In TWRP it’s done with Advanced - ADB Sideload.
If this doesn’t work for some reason, you could also transfer the ZIP file to the phone and install it from TWRP’s main menu with Install.
Hmmm … I don’t use the /e/ recovery, and I don’t have your device.
ADB Sideload needs to be started in the recovery first. I guess there should be some message on the phone that Sideload was started or that the phone would wait for file transfer now or something like this.
Then adb devices on the computer should report the serial number along with “sideload” to confirm the recovery really is in ADB Sideload mode.
Then execute the adb sideload command.
As said … TWRP’s Install does the same job, but it needs the ZIP file somewhere on the phone, either in Internal Storage or on an external SD card.