[4bpp]

The post, at least, seems to have no word about a web interface, though? I'm aware of a number of CalDAV/CardDAV servers, but they all seem to be designed for a workflow where you only access the calendar from a mobile app. Conversely, standalone web calendars like the fullcalendar.io thing linked in a sibling post make no mention of any support for CalDAV or other ways to synchronise with phones and calendar apps.

It's the integration of both that makes Google (and, I guess, Nextcloud) useful: you can add an event on your computer (which is where most of the scheduling and planning happens), and then inspect and be alerted of it on your phone (which is with you when you are in a random location and need to be reminded of an event).

Even if there does exist a standalone JS calendar application that can sync with CalDAV, you would be left with an awkward setup when self-hosting, since now for no obvious reason you need to have two services on the same machine (the database and the frontend) that maintain a copy of the same state and need to sync with each other constantly.