Their ebook is not available for purchase. They make it sound like that's the tradeoff, which is sad even more than ironic since of course there are alternatives.

I just checked, and this book (from 2016) is not available from a German online shop that carries a lot of DRM-free epubs, for example (buch7.de).

So, yet again, the only way to acquire an actual electronic copy that is not borrowed is via "piracy".