The instructions I posted are for updating the RPi/VideoCore firmware on the SD card / USB device. On Raspbian "rpi-update" takes care of that step. Without that updated firmware the bootloader will refuse to boot from USB media.
The bootloader eeprom on the RPi has to be updated separately to the beta version. This is done via "rpi-eeprom-update". While LibreELEC contains the rpi-eeprom-update/rpi-eeprom-config tools it doesn't include the beta bootloader - you'd need to manually download that from GitHub - raspberrypi/rpi-eeprom: Installation scripts and binaries for the closed sourced Raspberry Pi 4 EEPROMs . Or boot Raspbian on the RPi and follow the official instructions Raspberry Pi 4 USB mass storage beta (beta means it not ready yet, and not officially released!) - Raspberry Pi Forums
The same warning as in the RPi thread applies to LE, too - even more so as LE is a bit different than Raspbian.
QuoteThis is a beta release. If you aren't already comfortable with manual firmware updates then please wait until this is available in a standard release.
so long,
Hias