A couple of things. Writing a firmware update file to the µSD card is useless since it does not include all the bits and pieces needed. The µSD card has 3 partitions (2 Linux ext4, 1 FAT32 plus some free space which is used for boot loader, serial number, etc.)
What you need is the image for the entire µSD which you can request in the
Clara SD card image needed thread. Please note that the images are sent via PM and the second link in the PM is the how-to.
BTW, the forum you posted is for Kobo's old Android tablets not their eInk devices.
PM sent