If I had to guess I’d say that their other project, Sponserblock, got a little bit more popular than they were expecting and this is just to help alleviate server costs. Most of the API endpoints don’t require any auth at all (the single one that does accepts a random UUID), so any checks must be locally done (maybe system time?). The extension and server back-end are licensed under GPLv3 and AGPL respectively and are also entirely self-hostable, so the code is out there to verify if you wish
Depends
Clannad? Valid
After-Story?