5.9.4 is fairly recent.
There might be some changes present that these old directions do not allow for.
(and of course, you do not use the quotes in the:
"bootm 0xE41000" command)
PS: Did you check if your KT3* / 5.9.4 firmware includes ADBd?
If yes, somebody here more familiar than I with Android DeBug can give you directions on using it to install the single jail break file.
*
https://wiki.mobileread.com/wiki/Kindle_Serial_Numbers
Because, as you will notice, there are ten (10) "8th generation" Kindle models.
Please use our nickname for your device when posting here (and/or fix your left column info block).