Hidden linux folder?

Those aren't very hidden, it is just a Linux UI thing that assumes you don't want to see dotfiles.
There are a couple used with active content, which you will immediately see cluttering the userstore on any Windows computer.
(And, the userstore is fat32, which means no file permissions or immutable bits.)
Nope -- in fact, on my Kindle Touch they are all in [userstore]/documents/dictionaries/ and can be Removed From Device (and archived in the Cloud view) like any other book.
But it stands to reason they must be
somewhere in the userstore, because the alternative is that they are embedded in the read-only rootfs. Which would be worryingly stupid.
...
I don't remember when the last time I checked deleting dictionaries was. Probably a long time ago, back when, I guess, they still allowed you to.
I don't know why Amazon has now decided to stop you from deleting the default documents.
Presumably they still allow you to send the latest version from MYK? Although I don't know who would want an updated User Guide.