Ok, I misspelled it. It is
kindl, not kndl. Either way, it might work as a downloader, or might bork something, I don't know. Android emulator would work, but getting books off it might be a hassle. The blocked books are usually (but not always) pre-kfx (Mobi).
As to why, well, part of it is DRM, and part appears to be sheer sloth. Some older formats are just not supported well on their React app and they just... did not bother, and blocked them. Pretty sure you can read them in browser? (Did not try, though. ) That would be good enough for Amazon.