what you mean by locking down? your epub is your epub . i read third party epubs on kindle all the time. i am not sure if i am missing something here dont understand the downvotes. I've read third party books on kindle for decades with no issues.
I own two kobos, a libra for fiction and an ellipsa for tech books and textbooks. Both have koreader on.
No, you do not. Kindle has never and, as far as I am aware, to this day, does not support epubs. The only way to read epubs on your Kindle is to convert them either by Amazon's Send to Kindle service or through software like Calibre.
1. Epub of a book exists on a computer
2. User takes simple action to read that book on a Kindle device
3. ???
4. User succeeds at reading that same book
Like when Github invisibly renders Markdown into HTML for display in my browser: It seems rational to say that I'm reading a Markdown document.
Or when I print that same HTML onto paper and read it. I'm not reading the HTML nor the Postscript intermediates: I'm still reading a Markdown document.
I don't hate Amazon, but I was not willing any longer to give them knowledge and control over my reading.