What exactly did you try to reboot the phone?
adb reboot? The adb command doesn’t work in Fastboot Mode, only the fastboot command does.
I had a look at the steps, and I see a possible problem …
The “Installing a custom recovery” section ends with “Now reboot into recovery to verify the installation”.
But it does only tell you how to do that with the “Tip: On some devices if the device does not boot into TWRP you could try it manually by following these steps - With the device powered off, hold Volume Up + Power. Release when boot logo appears.”
This could be made clearer.
After you flashed the TWRP image, you have to reboot or start the phone while keeping Volume Up pressed to get into the recovery (which should be TWRP now). When you are in Fastboot Mode, you can reboot the phone by doing
fastboot reboot at this point, or you can force a reboot with the power button. Either way you’ll have to keep Volume Up pressed to get into the recovery.