The author wrote a bit more about making this app in Japanese here: https://note.com/kasamiworks/n/n69bc8d1cf943

What the landing page doesn't say: the free version is limited to 5 people only - such a small number that SRS is meaningless. The dev has made a big mistake here, building an extensive and polished product without first making an MVP to gauge product market fit. If he instead released a free MVP that could actually be used (unlimited people, maybe other features are limited) then maybe some people would actually use it. But as it is now probably anyone who downloads it is just going to delete it right away.