… this guide is totally correct and to be followed in detail.
A guide with a direct download link to 18.04.1 (in case the download on the support page switches to 18.09.2) is here (“I regret upgrading, how do I downgrade?”) …
You are instead advising to use an incremental 18.03.1 → 18.04.1 OTA update file intended for use with the Updater or the recovery … with the manual fastboot installation procedure for installing the whole OS … Good luck with that, indeed. It will not work.