Carefully read the comments attached to that video.
Do any of them sound familar now? All of them maybe.
(When that author first changed a file on the Kindle's fat32 file system and then unplugged the cable **without** doing the "safely remove" step was the time to close that video.)
You both have what is called a "K4" on this forum, but probably different firmware versions.
The quick and easy way out for you at this point: Use Kubrick on it to get back to the "factory stock" image + plus a few of the MR basics (all of which are harmless).
Note: Your device isn't actually "bricked" - but the process of recovery from that condition takes longer and more skill than to just use the Kubrick wizard.