Eh, I wouldn't be so sure. Reading the DMCA, their code does seem to do what the law says you can't do[1]:

    "No person shall circumvent a technological measure that effectively controls access to a work protected under this title [...]"

with these definitions[2]:

    (A) to “circumvent a technological measure” means to descramble a scrambled work, to decrypt an encrypted work, or otherwise to avoid, bypass, remove, deactivate, or impair a technological measure, without the authority of the copyright owner; and

    (B) a technological measure “effectively controls access to a work” if the measure, in the ordinary course of its operation, requires the application of information, or a process or a treatment, with the authority of the copyright owner, to gain access to the work.

I think (A) pretty clearly applies: the glyphs being randomized in each request obviously counts as being "scrambled", the method used by the author with the hashes clearly descrambles them by matching the provided SVG images to the letters rendered with the book's font.

I'm less sure about (B), not being a lawyer, but I think it's so generic that it does apply: the "ordinary course of [...] operation" of reading the book requires running the apps provided by Amazon. This seems to fit "requires the application of [...] a process [...] with the authority of the copyright owner".

[1] https://www.law.cornell.edu/uscode/text/17/1201

[2] https://www.law.cornell.edu/definitions/uscode.php?width=840...