You cannot -- Kindle ebooks are in a format called KFX (or AZW/mobi or AZW3, depending on when you got them), which this tool doesn't support as an input.
If you wanted to do that, I'd just use Calibre, which has really comprehensive format conversion abilities: https://calibre-ebook.com
Most Kindle ebooks have DRM as well, which I think is currently hard to remove -- there's a plugin for calibre that you could try. Some publishers don't apply it, though, so maybe.
This is a fantastic way to read books published by Tor on multiple devices. Now if only Tor would sell the ePub files on their website. (Tor eBooks are not DRMed.)
I'm an author on Amazon and wonder: Is this also true if I switch off DRM for my books on Amazon? Could someone clarify this? I publish all my books without DRM.
And indeed once you purchased his books you can download a DRM-free epub or pdf file of the book you purchased and use any tool you like to read it.
I don't see similar note on the listings of his books on Kindle store, so my _guess_ would be that Amazon is unwilling do fulfill this request from the author, but I could be wrong (I never owned any kindle device anyways).
The Kindle store doesn't make it obvious if something is DRM free anymore. The most obvious way is that DRM free books will usually list the number of simultaneous devices as unlimited while DRM books will not. Happy to be corrected if there is a nicer way of telling.
If you wanted to do that, I'd just use Calibre, which has really comprehensive format conversion abilities: https://calibre-ebook.com
Most Kindle ebooks have DRM as well, which I think is currently hard to remove -- there's a plugin for calibre that you could try. Some publishers don't apply it, though, so maybe.