Dont do that, you are bricking your LibreELEC installation.
Only replace start.elf and fixup.dat, eg with the script or instructions in the linked post, and make sure no other start/fixup files are present.
It may work for you now but if you have other start/fixup files on your SD card your RPi won't be using the one needed for LibreELEC and on later LibreELEC updates you'll still be using that old version which can cause a lot of headache for you.
USB boot is working fine here with the May 27 firmware and bootloader versions.
so long,
Hias