Paperwhite 2 Firmware 5.8.11 question: enhanced typeset available ONLY for AZW3?

So last night my Kindle Paperwhite 2 updated to 5.8.11. One of the new features is enhanced typesetting with different degrees of bold.

I was reading some .mobi books I had sent to my Kindle email address but the option to change the boldness of the text wasn't available.

Is sideloading a AZW3 the only way of getting these new features on a book not purchased from Amazon?

Thanks!