That turns out not to be true at all.
This is a post by the guy who created TPZ for Amazon: Interesting Topaz DRM development (geekery, job stuff)
It has absolutely no relationship to any other ebook format out there; it is not “mobipocket with fonts,” it is not “a proprietary implementation of ePub,” or any of the other ridiculous things I’ve seen written about it from people who have no idea but make these statements as if factual
The short version: TOPAZ is a series of scans formatted in such a way to provide reflow. I suppose something along the lines of reflowable PDFs now available.
It’s a very interesting post that should be read by every Kindle owner.
I think it’d be very rare to come across an eBook aimed at the mass market in TPZ format. It seems it was created for special formatting cases.