It's conflating a format for recording calendars with a syntax the user needs to write.

That syntax looks quite brittle and not very intuitive. One day = one line would get unwieldy fast. It doesn't localise well and there's no obvious way to implement recurring events beyond daily actions. (From what I can see at least?)

You could build a client on top of that but then, you lose the benefit.

It's a neat idea and it's good to see someone share a simple concept that works for them. But I don't see it working for most people.