newbie24689
You're right. The one calendar app is supposed to be a real functioning app, but its launcher was removed. It's the default calendar app included in Android's AOSP.
I did a search on Matrix and found out why (this is a quote from the lead developer):
GrapheneOS only includes AOSP Calendar to provide required intents used by other apps
What this means is it allows things like share an event from one app, or event notifications, etc. That can include associating a file extension with the app, even if the app doesn't have an icon. Removing the app completely would break some basic functionality.
The quote says why it's included but not why it's disabled. I'm guessing it's because the app is terrible and the GrapheneOS devs decided it's better if users find their own replacement.