The rules implemented are very simple (taken from the patent application [1], the exact numbers are configurable).
If there are <= 3 letters, one letter is bold.
If there are == 4 letters, two letters are bold.
If there are > 4 letters, 40% of all letters are bold.
There is this claim as the first text on the website:
We are happy if as many people as possible can use the advantage of Bionic Reading. For this reason, Bionic Reading should be able to be integrated into existing apps and services. The benefit for the reader should be the main focus.
If that is truly correct, why not publish these 3 rules? Why hide it behind multiple patent applications and trademarks? Why spend all this time and money on patents and an API that probably involve sending all text to some servers, just so that these 3 lines of code can be executed?
[1] https://patents.google.com/patent/DE102017112916A1/en
[1] https://patents.google.com/patent/DE102017112916A1/en