We do not write content with AI, in English or Japanese. All English content is written by native speakers of English and all Japanese content is written by native speakers of Japanese. Translated versions of Bunpro (French, etc.) are also translated by native speakers of those respective languages.
The only AI thing you may want to be aware of is the audio for vocab sentences. Grammar sentences only have human voices though. You can turn off that audio in settings though, if you would like to entirely avoid it.
I can guarantee Topics was written by humans (I am one of those humans). The majority of the Topic content is written by me, with editing and some bits and pieces being a group effort. The examples are written by natives on the team. The translations are done by native English speakers. Both are then checked and edited by others.
(I can tell you it isn’t AI since I think you can probably find typos and editing errors in the content if you look hard enough…)